When testing compare only the stdout

for jruby, there are warning lines are being output. This way, we can differentiate between stdout/stderr output and keep the tests focused on the actual output.

test/cases/engine_internals_test.rb

@@ -1,4 +1,4 @@
-
+require 'open3'
 require 'cases/test_base'
 
 class EngineInternalsTest < TestBase
@@ -37,9 +37,9 @@ def inspect
         print Haml::Engine.new('%div{ foo: true, bar: false }').render
       HAML
       f.close
-      out = IO.popen([RbConfig.ruby, f.path], &:read)
+      stdout_str, _stderr_str, _status = Open3.capture3("#{RbConfig.ruby} #{f.path}")
       assert_equal true, $?.success?
-      assert_equal "<div foo></div>\n", out
+      assert_equal "<div foo></div>\n", stdout_str
     end
   end