test/ruby/envutil.rb: compare RSS to check memory leak

* test/ruby/envutil.rb (Test::Test#assert_no_memory_leak): compare
  also RSS if available.

* test/ruby/memory_status.rb (Memory::Status.parse): read string
  form of an instance.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45298 b2dd03c8-39d4-4d8f-98ff-823fe69b080e