test: do not run on built-in

After embedding checks to the core binary, plain require('checks') will
return embedded module. So it won't be possible to run repository module
tests (in reusable_testing or with repository CI) with repository code
until it overloads build-in code. Until [1] is resolved there are no
any other way to deal with it. (And even after [1] would be resolved,
it may worth to leave it like this to support older versions with
built-in checks.)

1. tarantool/tarantool#7774

Part of tarantool/tarantool#7726

perftest.lua

@@ -1,14 +1,18 @@
 #!/usr/bin/env tarantool
 
-local t = require('luatest')
-
-local checks = require('checks')
 local json = require('json')
 local log = require('log')
 local uuid = require('uuid')
 
+local t = require('luatest')
+
 local g = t.group('checks_performance')
 
+-- Clean up built-in checks to run tests with repository module.
+package.preload['checks'] = nil
+package.loaded['checks'] = nil
+local checks = require('checks')
+
 local args = {
     ['nil'] = nil,
     ['string'] = 'str',

test.lua

@@ -1,12 +1,16 @@
 #!/usr/bin/env tarantool
 
-local t = require('luatest')
-
 local json = require('json')
-local checks = require('checks')
+
+local t = require('luatest')
 
 local g = t.group('checks')
 
+-- Clean up built-in checks to run tests with repository module.
+package.preload['checks'] = nil
+package.loaded['checks'] = nil
+local checks = require('checks')
+
 ------------------------------------------------------------------------------
 local current_file = debug.getinfo(1, 'S').short_src