Rework integration testing in sketch-file-format to account for new Sketch schema smoke test

[jedrichards]

At the moment sketch-file-format tests against a specific version of Sketch, but with the new testing strat the file format no longer needs knowledge of a specific Sketch version.

It might be useful however, to retain some sort of script in the file format repo that enables testing the schemas against an arbitrary Sketch file, for use during development.