Auto merge of #33588 - nikomatsakis:compiletest-ui, r=acrichto
add UI testing framework
This adds a framework for capturing and tracking the precise output of rustc, which allows us to check all manner of minor details with the output. It's pretty strict right now -- the output must match almost exactly -- and hence maybe a bit too strict. But I figure we can add wildcards or whatever later. There is also a script intended to make updating the references easy, though the script could make things a *bit* easier (in particular, it'd be nice if it would find the build directory for you automatically).
One thing I was wondering about is the best way to test colors. Since windows doesn't embed those in the output stream, this test framework can't test colors on windows -- so I figure we can just write tests that are ignored on windows and which pass `--color=always` or whatever to rustc.