Auto merge of #96551 - ferrocene:pa-ignore-paths-when-abbreviating, r=Mark-Simulacrum

[compiletest] Ignore known paths when abbreviating output

To prevent out of memory conditions, compiletest limits the amount of output a test can generate, abbreviating it if the test emits more than a threshold. While the behavior is desirable, it also causes some issues (like #96229, #94322 and #92211).

The latest one happened recently, when the `src/test/ui/numeric/numeric-cast.rs` test started to fail on systems where the path of the rust-lang/rust checkout is too long. This includes my own development machine and [LLVM's CI](https://github.com/rust-lang/rust/issues/96362#issuecomment-1108609893). Rust's CI uses a pretty short directory name for the checkout, which hides these sort of problems until someone runs the test suite on their own computer.

When developing the fix I tried to find the most targeted fix that would prevent this class of failures from happening in the future, deferring the decision on if/how to redesign abbreviation to a later date. The solution I came up with was to ignore known base paths when calculating whether the output exceeds the abbreviation threshold, which removes this kind of nondeterminism.

This PR is best reviewed commit-by-commit.

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