Use require_relative instead of require

Follow up faee2dd

This commit fixes the following error:

```console
$ bundle exec minitest-queue test/**/*.rb
Starting test-queue master (/tmp/test_queue_5033_1580.sock)
/Users/koic/src/github.com/rubocop/rubocop-minitest/test/rubocop/cop/minitest/no_test_cases_test.rb:3:
in `require': cannot load such file -- test_helper (LoadError)
        from /Users/koic/src/github.com/rubocop/rubocop-minitest/test/rubocop/cop/minitest/no_test_cases_test.rb:3:
        in `<top (required)>'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner/minitest5.rb:87:in `require'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner/minitest5.rb:87:in `suites_from_file'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:332:
        in `block (2 levels) in discover_suites'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:331:in `each'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:331:in `block in discover_suites'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:325:in `fork'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:325:in `discover_suites'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:243:in `execute_internal'
        from /Users/koic/src/github.com/tmm1/test-queue/lib/test_queue/runner.rb:135:in `execute'
        from /Users/koic/src/github.com/tmm1/test-queue/exe/minitest-queue:8:in `<top (required)>'
```

test/rubocop/cop/minitest/no_test_cases_test.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'test_helper'
+require_relative '../../../test_helper'
 
 class NoTestCases < Minitest::Test
   def test_registers_offense_for_empty_test_class