Update E2E test harness to verify specific failures.

[otrho]

I think we've talked about this but I can't find a specific issue tracking it -- the should_fail tests should only 'pass' when the failures (errors and/or warnings) are specifically what we expect. At the moment the projects could fail to build for any reason and the tests would still pass.

[mohammadfawaz]

Slightly related #723

But absolutely.. this would be extremely helpful. I'd like to be able to write one negative test that checks for many error messages. Preferably, those checks would live inside the test itself and not in mod.rs. E.g. from Clang: https://github.com/llvm/llvm-project/blob/47258ffc5c7aa9ae2017bdde58a88e0f928d8f07/clang/test/Parser/brackets.cpp#L10`