Reserve keyword arguments for new options in assert_template_result

[dylanahsmith]

Problem

As mentioned in #1611

The test/integration folder is meant to be a set of tests that can be shared with other liquid implementations, initially used with liquid-c. However, it still has coupling to the liquid library API, which could be further reduced.

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.

However, we can't just add keyword arguments to assert_template_result without calls to it that pass assigns as keyword arguments.

Solution

Update calls to assert_template_result that pass assigns as keyword arguments and instead pass a hash explicitly.

I also turned the message positional argument to assert_template_result into a keyword argument, which also reserves the ability to add additional keyword arguments without breaking its callers.

I also removed an unused assert_template_result_matches method and removed an unnecessary assigns argument to assert_match_syntax_error