test: Use require_relative for test helpers files

[cosmo0920]

Signed-off-by: Hiroshi Hatake hatake@clear-code.com

Which issue(s) this PR fixes:
None.

What this PR does / why we need it:

Without this patch, we need to run testcases with bundle exec rake test or additional TEST_OPTS environment variable to run individual testcases.

~\Documents\GitHub\fluentd [master ≡ +6 ~0 -0 !]> bundle exec ruby .\test\plugin\test_bare_output.rb
Traceback (most recent call last):
        3: from ./test/plugin/test_bare_output.rb:1:in `<main>'
        2: from ./test/plugin/test_bare_output.rb:1:in `require_relative'
        1: from C:/Users/cosmo/Documents/GitHub/fluentd/test/helper.rb:52:in `<top (required)>'
C:/Users/cosmo/Documents/GitHub/fluentd/test/helper.rb:52:in `require': cannot load such file -- helpers/fuzzy_assert (LoadError)

ruby /path/to/testcases_script.rb should run properly.

Docs Changes:

No need.

Release Note:

Same as title.