test: use luatest modules instead of internal ones

Some internal modules have been recently copied to luatest repo [1,2,3]
and now they can be safely removed, and the corresponding functionality
from luatest can be used instead.

Affected modules:

- test/luatest_helpers/misc.lua
- test/luatest_helpers/fiber.lua
- test/luatest_helpers/proxy/*

[1] https://github.com/tarantool/luatest/pull/247
[2] https://github.com/tarantool/luatest/pull/248
[3] https://github.com/tarantool/luatest/pull/255

Closes tarantool/luatest#238
Closes tarantool/luatest#251

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff

(cherry picked from commit 54bd77ad)

test/box-luatest/alter_with_compression_test.lua

-local misc = require('test.luatest_helpers.misc')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
 
@@ -17,7 +16,7 @@ g.after_all(function(cg)
 end)
 
 g.test_invalid_compression_type_during_space_creation = function(cg)
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     cg.server:exec(function(engine, compression)
         local t = require('luatest')
         local format = {{
@@ -36,7 +35,7 @@ g.before_test('test_invalid_compression_type_during_setting_format', function(cg
 end)
 
 g.test_invalid_compression_type_during_setting_format = function(cg)
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     cg.server:exec(function(compression)
         local t = require('luatest')
         local format = {{

test/box-luatest/gh_7005_session_on_commit_rollback_test.lua

-local misc = require('test.luatest_helpers.misc')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
 local g = t.group()
@@ -63,7 +62,7 @@ end
 -- Checks session and user in box.on_rollback trigger callback in case
 -- transaction is rolled back on WAL error.
 g.test_session_on_wal_error_rollback = function()
-    misc.skip_if_not_debug()
+    t.tarantool.skip_if_not_debug()
     g.server:exec(function()
         local t = require('luatest')
         box.session.su('eve', function()

test/box-luatest/gh_7583_txn_stat_test.lua

-local misc = require('test.luatest_helpers.misc')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
 
@@ -131,7 +130,7 @@ g.after_test('test_wal_error', function(cg)
 end)
 
 g.test_wal_error = function(cg)
-    misc.skip_if_not_debug()
+    t.tarantool.skip_if_not_debug()
     cg.server:exec(function()
         local t = require('luatest')
         local function stat()

test/box-luatest/space_upgrade_test.lua

-local misc = require('test.luatest_helpers.misc')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
 local g = t.group()
@@ -16,7 +15,7 @@ g.after_all = function()
 end
 
 g.test_low_level_api = function()
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     g.server:exec(function()
         local t = require('luatest')
         t.assert_error_msg_equals(
@@ -27,7 +26,7 @@ g.test_low_level_api = function()
 end
 
 g.test_high_level_api = function()
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     g.server:exec(function()
         local t = require('luatest')
         t.assert_error_msg_equals(

test/box-luatest/transport_test.lua

-local misc = require('test.luatest_helpers.misc')
 local net = require('net.box')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
@@ -48,7 +47,7 @@ g.test_net_box = function()
 end
 
 g.test_listen_ssl = function()
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     g.server:exec(function()
         local t = require('luatest')
         t.assert_error_msg_equals(
@@ -58,7 +57,7 @@ g.test_listen_ssl = function()
 end
 
 g.test_replication_ssl = function()
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     g.server:exec(function()
         local t = require('luatest')
         t.assert_error_msg_equals(
@@ -68,7 +67,7 @@ g.test_replication_ssl = function()
 end
 
 g.test_net_box_ssl = function()
-    misc.skip_if_enterprise()
+    t.tarantool.skip_if_enterprise()
     t.assert_error_msg_equals(
         'SSL is not available in this build',
         net.connect, {g.server.net_box_uri, params = {transport = 'ssl'}})

test/luatest_helpers/misc.lua

-local luatest = require('luatest')
-local tarantool = require('tarantool')
-
--- Returns true if Tarantool build type is Debug.
-local function is_debug_build()
-    return tarantool.build.target:endswith('-Debug')
-end
-
--- Returns true if Tarantool package is Enterprise.
-local function is_enterprise_package()
-    return tarantool.package == 'Tarantool Enterprise'
-end
-
--- Skips a running test unless Tarantool build type is Debug.
-local function skip_if_not_debug()
-    luatest.skip_if(not is_debug_build(), 'build type is not Debug')
-end
-
--- Skips a running test if Tarantool package is Enterprise.
-local function skip_if_enterprise()
-    luatest.skip_if(is_enterprise_package(), 'package is Enterprise')
-end
-
-return {
-    skip_if_not_debug = skip_if_not_debug,
-    skip_if_enterprise = skip_if_enterprise,
-}

test/vinyl-luatest/gh_7240_abort_yielding_dml_test.lua

-local misc = require('test.luatest_helpers.misc')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
 
@@ -28,7 +27,7 @@ end)
 --    because it's in read view.
 --
 g.test_abort_yielding_dml = function(cg)
-    misc.skip_if_not_debug()
+    t.tarantool.skip_if_not_debug()
     cg.server:exec(function()
         local fiber = require('fiber')
         local t = require('luatest')