[ruby/irb] Change show_source tests into integration tests

* Remove trailing spaces

* Migrate show_source tests to integration tests

Because show_source tests often need to define class and/or methods,
they can easily leak state to other tests. Changing them to integration
tests will ensure that they are run in a clean environment.

* Fix NoMethodError caused by SourceFinder#method_target

3c39f13397

lib/irb/source_finder.rb

@@ -77,6 +77,8 @@ module IRB
         target_method = target_method.super_method if target_method
       end
       target_method.nil? ? nil : target_method.source_location
+    rescue NameError
+      nil
     end
   end
 end