Enable checkers on parallel test.

parallel test (`make test-all TESTS=-j8`) runs tests on specified
number of processes. However, some test checkers written in
`runner.rb` are not loaded. This fix enable these checkers on
parallel tests.

See also: https://github.com/ruby/ruby/pull/2508

tool/lib/iseq_loader_checker.rb

@@ -39,7 +39,11 @@ class RubyVM::InstructionSequence
   end
 
   opt = ENV['RUBY_ISEQ_DUMP_DEBUG']
+
+  if opt && caller.any?{|e| /test\/runner\.rb/ =~ e}
     puts "RUBY_ISEQ_DUMP_DEBUG = #{opt}" if opt
+  end
+
   CHECK_TO_A      = 'to_a'      == opt
   CHECK_TO_BINARY = 'to_binary' == opt
 

tool/lib/test/unit/parallel.rb

@@ -2,6 +2,11 @@
 $LOAD_PATH.unshift "#{File.dirname(__FILE__)}/../.."
 require 'test/unit'
 
+require "profile_test_all" if ENV.key?('RUBY_TEST_ALL_PROFILE')
+require "tracepointchecker"
+require "zombie_hunter"
+require "iseq_loader_checker"
+
 module Test
   module Unit
     class Worker < Runner # :nodoc:

tool/test/runner.rb

@@ -5,10 +5,10 @@ $LOAD_PATH.unshift File.expand_path("../lib", __dir__)
 
 require 'test/unit'
 
-require_relative "../lib/profile_test_all" if ENV.key?('RUBY_TEST_ALL_PROFILE')
-require_relative "../lib/tracepointchecker"
-require_relative "../lib/zombie_hunter"
-require_relative "../lib/iseq_loader_checker"
+require "profile_test_all" if ENV.key?('RUBY_TEST_ALL_PROFILE')
+require "tracepointchecker"
+require "zombie_hunter"
+require "iseq_loader_checker"
 require_relative "../test-coverage.rb" if ENV.key?('COVERAGE')
 
 case $0