Create two different tools:

  - `test-drive`: A rustc_driver that compiles a crate and run a few
  sanity checks on StableMIR.
  - `compiletest`: A wrapper to run compiler tests using the
    `test-drive` tool.

I am also adding a script to run a few rustc tests and a nightly
workflow. The files diff is not quite working yet so most tests
that fail compilation don't succeed yet.

I had forgotten to enable the smir checks before. Unfortunately,
enabling them triggered a few issues.

For now, I set the rustc suite to not fail the CI and created issue
rust-lang#39 to track
the work needed to enable it.