Cross-check rcc against other compilers

[jyn514]

We do this at work for our SQL implementation: https://gitlab.com/YottaDB/DBMS/YDBOcto/-/blob/master/tests/test_helpers.bash.in#L551. The basic idea is to run two compilers on the same code and make sure they agree. We could integrate this with existing assert_compiles, etc. tests and make sure that we're testing the right thing. This would also let us take advantage of the existing test suite without much work.

Inspired by @pythondude325 finding that fn redefinition was testing the wrong thing (#470) - the test passed, but it shouldn't have.

[pythongirl325]

This is effectively what I do by testing hexponent against libc's equivalent functions.

[pythongirl325]

Could we do this very naively by just using cc to write the tests?

[jyn514]

The way I was planning to do this is modifying

saltwater/tests/utils/mod.rs

Line 75 in 8a52d91

pub fn assert_compiles(program: &str, path: PathBuf) {

, etc. to run cc against the code in addition to swcc, yeah.