Support testing under different editions

It's occasionally useful to test that that macros work correctly regardless of the call site edition, since plenty of behavior depends on the edition of the main crate rather than the macro crate. Does this seem appropriate for trybuild?

I think I'd be happy if `TestCases` had a method to configure the edition to run tests under. Though I wonder if using the same `.stderr` file for effectively multiple tests is a good idea -- could we maybe compare the output with `.editionN.stderr` if it exists and `.stderr` otherwise?