Rollup merge of #121373 - Zalathar:test-revision, r=oli-obk

Consistently refer to a test's `revision` instead of `cfg`

Compiletest allows a test file to specify multiple “revisions” (`//@ revisions: foo bar`), with each revision running as a separate test, and having the ability to define revision-specific headers (`//`@[foo]` ignore-blah`) and revision-specific code (`#[cfg(foo)]`).

The code that implements this feature sometimes uses the term “cfg” instead of “revision”. This results in two confusingly-different names for the same concept, one of which is ambiguous with other kinds of configuration (such as compiletest's own config).

This PR replaces those occurrences of `cfg` with `revision`, so that one name is used consistently.

Author: matthiaskrgr