test/config: fix helper to use w/o extra config options

A set of testing helpers was added in commit 06ca83c9 ("test/config:
add several application script tests"). In particular, they allow to run
a tarantool instance with some default config file and write only
necessary config options in a test case.

In fact, if no options were set in the test, the config file was not
written. It is fixed in this commit and used in the next commit.

Part of #8862

NO_DOC=testing helper change
NO_CHANGELOG=see NO_DOC

test/config-luatest/helpers.lua

@@ -102,15 +102,17 @@ local function prepare_case(g, opts)
         treegen.write_script(dir, 'main.lua', script)
     end
 
+    local config = simple_config
     if options ~= nil then
-        local config = table.deepcopy(simple_config)
+        config = table.deepcopy(simple_config)
         for path, value in pairs(options) do
             cluster_config:set(config, path, value)
         end
-        treegen.write_script(dir, 'config.yaml', yaml.encode(config))
     end
 
+    treegen.write_script(dir, 'config.yaml', yaml.encode(config))
     local config_file = fio.pathjoin(dir, 'config.yaml')
+
     local server = {
         config_file = config_file,
         chdir = dir,
@@ -198,6 +200,7 @@ end
 --   Verify test invariants after config:reload().
 local function reload_success_case(g, opts)
     local script_2 = assert(opts.script_2)
+    local options = assert(opts.options)
     local verify_2 = assert(opts.verify_2)
 
     local prepared = success_case(g, opts)
@@ -205,6 +208,7 @@ local function reload_success_case(g, opts)
     prepare_case(g, {
         dir = prepared.dir,
         script = script_2,
+        options = options,
     })
     g.server:exec(function()
         local config = require('config')
@@ -232,6 +236,7 @@ end
 --   An error that config:reload() must raise.
 local function reload_failure_case(g, opts)
     local script_2 = assert(opts.script_2)
+    local options = assert(opts.options)
     local exp_err = assert(opts.exp_err)
 
     local prepared = success_case(g, opts)
@@ -239,6 +244,7 @@ local function reload_failure_case(g, opts)
     prepare_case(g, {
         dir = prepared.dir,
         script = script_2,
+        options = options,
     })
     t.assert_error_msg_equals(exp_err, g.server.exec, g.server, function()
         local config = require('config')