Auto merge of #128916 - jieyouxu:dump-ice-dump-ice, r=compiler-errors

Tidy up `dump-ice-to-disk` and make assertion failures dump ICE messages

For the future traveler: **if you did a `git blame` and found this PR that last modified `dump-ice-to-disk` because the test failed in a completely unrelated PR, then I'm afraid our ICE dump may have regressed or somehow behaves differently on `i686-mingw`.**

A bit of clean up to the `dump-ice-to-disk` test.

- Fixes/updates the top-level comment.
- Add a FIXME pointing to #128911 for flakiness.
- Instead of trying to manually cleanup `rustc-ice*.txt` dumps, run each test instance in a separate temp directory.
- Explicitly make `RUSTC_ICE` unavailable in one of the `-Zmetrics-dir` test case to not have interference from environment.
- Make assertion failures (on ICE dump line count mismatch) extremely verbose to help debug why this test is flakey in CI (#128911).

Contains a fix by `@saethlin` in #128909, should wait until that is merged then rebase on top.

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: x86_64-msvc
try-job: i686-mingw
try-job: test-various

Author: bors