Rollup merge of #148065 - Zalathar:test-paths, r=jieyouxu

compiletest: Add concrete examples for some config/test path fields

Seeing a specific example path can be much more enlightening than trying to figure out what the prose is gesturing towards.

Also, in some cases the existing comments were incorrect or misleading, as demonstrated by the example paths.

The example paths were determined by dumping them directly out of the config with `dbg!`, and then lightly anonymizing them for example purposes.

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No functional changes.

r? jieyouxu

Author: jhpratt