Filenames include simulator details

Provides #182

Proof-of-concept for a heading check, this is far from complete.

Still to do: * [ ] Everything except iOS-on-Mac is new to me: MacOS, tvOS, Linux, etc need attention * [ ] All FIXMEs removed * [ ] Moar tests * [ ] Make the simulators used for CI explicit (because they matter for the filenames) * [ ] Documentation * [ ] Another PR for v2, because all the test-snapshot filenames will need updating...