Switch compile-fail tests to ui tests

[RalfJung]

Currently, we use the compile-fail mode of compiletest for our "expected to fail" tests. rustc has switched to ui a long time ago, and it probably would make sense for Miri to follow suit. That would also unblock #2015.

#1977 finally makes this feasible by hiding libstd internals from the default stacktraces.

However, besides making sure we can actually get compiletest to do this properly (miri's needs are somewhat special), we also need to figure out how to support something like --bless. Clippy might be a good template here.

ui tests would also make it much easier to evaluate PRs like #1971.

[RalfJung]

Thanks a lot @oli-obk for doing this. :-)