cargotest: replace Servo with standalone Stylo

[HNO3Miracle]

The Servo entry in cargotest pins a 2019 checkout and only tests the
selectors package. Cargo still has to resolve the old Servo workspace,
including its large graph of Git dependencies, even though most of that
workspace is not covered by the test.

Both selectors and the stylo CSS engine now live in the standalone
servo/stylo repository. This PR replaces
the old Servo checkout with a recent Stylo revision and runs both
cargo test -p selectors and cargo test -p stylo from the same checkout.

Stylo's unit tests previously failed under the default Servo configuration
because shared tests used pseudo-elements that were only available in Gecko.
This was fixed in servo/stylo#436,
and this PR pins the resulting revision.

The cargotest documentation is updated to list Stylo instead of Servo.

Tests:

• cargo test --manifest-path src/tools/cargotest/Cargo.toml
• target/debug/cargotest "$(command -v cargo)" /tmp/cargotest-stylo stylo
• ./x test tidy --set build.submodules=false

Closes #79404.

[rustbot]

rustc-dev-guide is developed in its own repository. If possible, consider making this change to rust-lang/rustc-dev-guide instead.

cc @BoxyUwU, @tshepang

[rustbot]

r? @clubby789

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[clubby789]

The full Stylo package is checked rather than tested because its current unit
tests do not compile under the default Servo configuration

What's the problem? Is this something we can address?

[HNO3Miracle]

This was tracked in servo/stylo#308. The shared tests used Gecko pseudo-elements such as MozRubyText and FirstLine, which do not exist under the default Servo configuration.

Coincidentally, this was fixed upstream yesterday by servo/stylo#436, which also enabled cargo test --workspace in Stylo CI.

I have updated this PR to use the resulting revision (127b0b5cab6a6927552e889debb20beb031b79d1) and test both packages directly with cargo test -p selectors -p stylo. Locally, all 13 selectors tests and all 8 stylo tests pass. This also allowed me to remove the added cargo check support.