Migrate all ParserTest fixtures to the gherkin testdata format

[stof]

Now that we have support for using custom tests written using the cucumber gherkin testdata format, all our existing fixtures used by ParserTest should be converted to the gherkin testdata format.

This will unblock #362 (and might unblock contributing some cases upstream to increase coverage)

[acoulton]

Thanks @stof - I had the same thought just now as well and was just about to post it 😆

~I'd also like to generate gherkin-testdata-format expectations for the upstream examples we know we parse differently (rather than just skipping them in legacy mode) which might allow us to delete some of our own examples already and will give better confidence that we've not changed any legacy mode parsing.~ (this bit done slightly differently in #392)