Compiletest: pass -Zui-testing to rustc

[Firestar99]

Supersedes #439, see discussions over there

• pass -Zui-testing to rustc to improve compiletest normalization and have fewer conflicts
• review commit by commit

[nnethercote]

I would squash together the 2nd and 3rd commits because they don't make sense in isolation (tests will fail if just the 2nd commit is applied). Otherwise, I'm not a rust-gpu reviewer, but LGTM.

[Firestar99]

I was considering it, but didn't as it "hides" the "-Zui-testing flag added" change. Whatever, let's squash them

[nnethercote]

"hides" the "-Zui-testing flag added" change

It's mentioned in the first line of the commit message! :)

[LegNeato]

Sweet! Thanks for doing this.

[eddyb]

To be perfectly honest, seeing this diff, the fact that we weren't using -Z ui-testing for the kind of tests named ui because of rustc testing infrastructure, is quite the facepalm moment 🤦.

Thanks for cleaning this up!