Support testing under different editions #307
Description
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?