Add new fields for internal test crate

[tamird]

Adjusting to changes made in:

• https://github.com/rust-lang/rust/pull/100451
• https://github.com/rust-lang/rust/pull/108148

[tamird]

Hmm, I see a bunch of errors of the form

error: crate `rustc_session` required to be available in rlib format, but was not found in this form
  |
  = help: try adding `extern crate rustc_driver;` at the top level of this crate

Thoughts?

[Manishearth]

Unsure, we haven't been trying hard to keep the nightly-feature stuff working.

It's possible the rustc internal components are distributed separately now.

I'll still merge this since it's an improvement.