Support `cargo dev bless` for tests with revisions
changelog: internal: Support `cargo dev bless` for tests with revisions
Previously bless wouldn't pick up the saved stderr from `target/debug/tests/manual_assert.stage-id.edition2021.stderr` or `target/debug/tests/manual_assert.stage-id.edition2018.stderr` due to there being multiple revisions of the test output
This tweaks compile-test so the built files end up in e.g. `target/debug/tests/ui`, `target/debug/tests/ui-cargo` rather than share the `tests` dir. `cargo dev bless` then uses that to update all the `.stdout/stdout/fixed` files it can find
Also removes an empty file I found, and the logic to remove empty outputs as compiletest doesn't produce empty `.stdout/stderr` files
caeebd6b09