require -> require_relative for tests

Makes it easier to run tests individually.

test/domain_test.rb

@@ -1,4 +1,4 @@
-require 'test_helper'
+require_relative 'test_helper'
 
 module GitHubLdapDomainTestCases
   def setup

test/filter_test.rb

@@ -1,4 +1,4 @@
-require 'test_helper'
+require_relative 'test_helper'
 
 class FilterTest < Minitest::Test
   class Subject; include GitHub::Ldap::Filter; end

test/group_test.rb

@@ -1,4 +1,4 @@
-require 'test_helper'
+require_relative 'test_helper'
 
 class GitHubLdapGroupTest < GitHub::Ldap::Test
   def self.test_server_options

test/ldap_test.rb

@@ -1,4 +1,4 @@
-require 'test_helper'
+require_relative 'test_helper'
 
 module GitHubLdapTestCases
   def setup

test/posix_group_test.rb

@@ -1,4 +1,4 @@
-require 'test_helper'
+require_relative 'test_helper'
 
 class GitHubLdapPosixGroupTest < GitHub::Ldap::Test
   def self.test_server_options