Auto merge of #1409 - RalfJung:compiletest-no-rustc, r=RalfJung
Auto merge of #1409 - RalfJung:compiletest-no-rustc, r=RalfJung

compiletest: no need to call rustc here

This also means we do not need the `RUSTC_TEST_SUITE` env var any more.

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri