Use compiletest_rs to confirm that the appropriate test cases don't compile

[jeremydavis519]

Currently, compiletest_rs doesn't expand macros, so we can't use it. I've requested that feature in Manishearth/compiletest-rs/issues/145. For now, a possible workaround is to update runtime-macros to allow failure to parse (which I probably should have done in the first place).