skipping test_backtrace_tweaking if Test::Unit::TestResult is not def…

…ined

Ruby 1.9.1's bundled test/unit, which is only a "test/unit compatibility
layer using minitest" does not include Test::Unit::TestResult, so this
can be skipped.

The skipping-message looks quite ugly, though.

spec/rr/test_unit/test_unit_backtrace_test.rb

@@ -15,6 +15,7 @@ def test_trim_backtrace_is_set
   end
 
   def test_backtrace_tweaking
+    skip "Skipped, because there is no Test::Unit::TestResult in this Test::Unit. Don't worry, this is totally fine." unless defined?(Test::Unit::TestResult)
     old_result = @_result
     result = Test::Unit::TestResult.new