Add partials, error_mode and render_errors options to assert_template_result

[dylanahsmith]

Follow-up to #1611

There were tests that couldn't be converted with the current assert_template_result & assert_match_syntax_error interface, since they require additional options to be supported. For instance, there is a need to specify the file system, error mode and to render liquid errors. Once that is done, more integration tests can be decoupled from the library API.

The partials, error_mode and render_errors options added by this PR correspond that stated need. I've also changed a few tests to actually use those options to exercise the code paths for the new options.