test: fix luacheck warnings in test/xlog

Closes #5468

Reviewed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Reviewed-by: Igor Munkin <imun@tarantool.org>

Co-authored-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Co-authored-by: Igor Munkin <imun@tarantool.org>

.luacheckrc

@@ -46,7 +46,7 @@ exclude_files = {
     "test/var/**/*.lua",
     "test/vinyl/**/*.lua",
     "test/wal_off/**/*.lua",
-    "test/xlog/**/*.lua",
+    "test/xlog/*.test.lua",
     "test/xlog-py/**/*.lua",
     "third_party/**/*.lua",
     ".rocks/**/*.lua",

test/xlog/panic.lua

 #!/usr/bin/env tarantool
-os = require('os')
+local os = require('os')
 
 box.cfg{
     listen              = os.getenv("LISTEN"),

test/xlog/upgrade/2.1.3/gh-4771-upgrade-sequence/fill.lua

 box.cfg{}
-s1 = box.schema.create_space('test1')
-pk = s1:create_index('pk', {sequence = true})
+local s1 = box.schema.create_space('test1')
+s1:create_index('pk', {sequence = true})
 s1:replace{box.NULL}
 
-seq2 = box.schema.sequence.create('seq2')
-s2 = box.schema.create_space('test2')
-pk = s2:create_index('pk', {sequence = 'seq2'})
+box.schema.sequence.create('seq2')
+local s2 = box.schema.create_space('test2')
+s2:create_index('pk', {sequence = 'seq2'})
 s2:replace{box.NULL}
 
-seq3 = box.schema.sequence.create('seq3')
+local seq3 = box.schema.sequence.create('seq3')
 seq3:next()
 
 box.snapshot()

test/xlog/xlog.lua

 #!/usr/bin/env tarantool
-os = require('os')
+local os = require('os')
 
 box.cfg{
     listen              = os.getenv("LISTEN"),