Generating and rebaslining -checks.js files should be automated

[dstoc]

do-magic.py auto-test-foo.html

the configuration could be part of the auto-test-foo.html definition:

autoTest(selector, props);

[mithro]

I'm actually slightly negative about this idea but willing to be convinced otherwise.

Generally making a change which causes rebaseline multiple tests seems to indicate;

• Our tests are highly codependent
• Our tests have a large amount of redundancy
• Our tests are fragile

I'm hoping my precision change will make rebaseline only occur when you change something drastically, then it makes sense that you should actually look at the tests verify the new checks files.

[dstoc]

This is only a nice-to-have. There's still the pain of generating the expectations in the first place.

This would also make it trivial to fix https://github.com/web-animations/web-animations-js/issues/355