Get the test suite working inside the rustc test suite

[oli-obk]

I had to weave some env vars over from rustc, but it works now both within rustc and here.

[RalfJung]

What exactly is it that used to not work and works with this? I don't understand what "inside the rustc test suite" means.

[RalfJung]

Ah, I see. This is for rust-lang/rust#43628. Looks good!

[RalfJung]

Of course, eventually, we will want to run miri on the entire rustc testsuite itself, not just on in its rather small testsuite. :)

[oli-obk]

Of course, eventually, we will want to run miri on the entire rustc testsuite itself, not just on in its rather small testsuite. :)

for that we need a way to mark tests as "miri will always fail on this"