Rollup merge of #143823 - jieyouxu:compiletest-maintenance-5, r=Kobzol

[COMPILETEST-UNTANGLE 5/N] Test mode adjustments and other assorted cleanups

This is part of a patch series to untangle `compiletest` to hopefully nudge it towards being more maintainable.

This PR should contain no functional changes modulo the removed debugger version warning.

- Commit 1: Removes a very outdated debugger version warning.
- Commit 2: Moves `string_enum` out of `common` into `util` module.
- Commit 3: Remove `#[derive(Default)` for `Mode` and `Config`. It is very important for correctness that we *don't* `#[derive(Default)]`, because there are no sensible defaults, so stop pretending there is.
- Commit 4: Rename `Mode` -> `TestMode`, because I would like to introduce a `TestSuite` enum to stop using stringly-typed test suite names where test mode names can be the same as test suite names, and we also have a bunch of other "modes" in compiletest. Make this as unambiguous as possible. A corollary is that now it's more natural to reference via intra-doc links as ``[`TestMode`]``.
- Commit 5: Ditto on `TestSuite`, stop glob-reexporting `TestMode::*` variants, and always use `EnumName::VariantName` form.
- Commit 6: Apparently, `src/tools/rustdoc-gui-test/` depends on `compiletest` for `//@ {compile,run}-paths` directive parsing and extraction, which involves creating a dummy `compiletest` config (hence the existence of the default impls removed in Commit 3). Make this a specific associated function with a FIXME pointing to rust-lang/rust#143827 as I think this setup is quite questionable.

Commits {4, 5} are also intended to help improve the self-consistency in nomenclature used within compiletest.

Best reviewed commit-by-commit.

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