[ruby/mmtk] Use RUBY_TEST_TIMEOUT_SCALE for tests

RUBY_TEST_TIMEOUT_SCALE is set for debug builds because they are slower
to run. We should respect this environment variable in MMTk tests too.

https://github.com/ruby/mmtk/commit/0a66c518bf

test/mmtk/helper.rb

@@ -9,9 +9,18 @@ module MMTk
 
     def setup
       omit "Not running on MMTk" unless using_mmtk?
+
+      @original_timeout_scale = EnvUtil.timeout_scale
+      timeout_scale = ENV["RUBY_TEST_TIMEOUT_SCALE"].to_f
+      EnvUtil.timeout_scale = timeout_scale if timeout_scale > 0
+
       super
     end
 
+    def teardown
+      EnvUtil.timeout_scale = @original_timeout_scale
+    end
+
     private
 
     def using_mmtk?

test/mmtk/test_configuration.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
+
 require_relative "helper"
+
 module MMTk
   class TestConfiguration < TestCase
     def test_MMTK_THREADS