GOAL: pleasant to use as part of a compiler's testsuite

[Gankra]

This is a metabug to track things needed for abi-cafe to be pleasant to incorporate into a compiler's testsuite.

• Add proper out-of-line test expectation files #9
• why the fuck did i fork cc-rs? (make us cargo-install-able) #49
• more usable minimizer output? #38
• How to mark test as busted? #65
• a more clear story on "use this toolchain for c/rust/..."?

[Gankra]

cc @bjorn3, if you have any asks you want to make, or issues you want to raise!

[Gankra]

One thing not noted here because I've put a bunch of work into improving it: a lot of the existing reasons you would need to checkout the entire codebase are improved:

• All the builtin test files are statically included in the binary
• We accept a cli flag for runtime inclusion of additional test files
• We no longer need procgen output to be checked in
• We no longer write generated source to random dirs, it's all in target

Test expectations and cc-forking are the last big holdouts.