envutil.rb: under_gc_stress parameter

* test/ruby/envutil.rb (EnvUtil#under_gc_stress): add stress parameter
  so that making possible to disable stressing explicitly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43625 b2dd03c8-39d4-4d8f-98ff-823fe69b080e