Don't report broken absolute path when app_root is nil

When running tests that does not initialize rails (e.g. `rake test` under
active_model), it used to just cut off the leading "/" from absolte paths and
report something like

rails test Users/a_matsuda/rails/activemodel/test/cases/api_test.rb:40

that cannot be executed.

railties/lib/rails/test_unit/reporter.rb

@@ -52,7 +52,11 @@ def filtered_results
     end
 
     def relative_path_for(file)
-      file.sub(/^#{app_root}\/?/, "")
+      if app_root
+        file.sub(/^#{app_root}\/?/, "")
+      else
+        file
+      end
     end
 
     private