Add a way of writing simple regression tests

[Marwes]

This adds the simplest possible regression test to corrode which only runs a c file through gcc as well as corrode + rustc and then runs both applications checking that their exit status is the same.

Also made some tweaks to corrode-cc to make it work for me on windows (in git bash)

[Marwes]

Potentially I could list all .c files in tests/regression-tests and generate a test for each file. What do you think @jameysharp ?

[Marwes]

Turns out that having main omit the return statement is only defined on c99 so I enabled c99 on the tests. I could perhaps just run the no-return-main under c99 but I am not aware of anything that needs to run under an older c version at the moment.