Rollup merge of #148199 - Zalathar:paths, r=jieyouxu

compiletest: Don't modify `testpaths` when creating aux contexts

Modifying `TestCx::testpaths` makes it vastly harder to reason about how compiletest is using test paths, and can be avoided without much trouble by simply passing explicit source-file paths into a small number of auxiliary-aware methods.

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This PR was originally based on a much larger change that split `TestPaths` into multiple structs and multiple `TestCx` fields (as seen in early revisions of rust-lang/rust#148160), but after further cleanups it turned out that very few tweaks were actually needed to avoid changes to `testpaths`.

r? jieyouxu

Author: matthiaskrgr