servo: Merge #19476 -  Use workspace.default-members to specify default crates for cargo build (from servo:default-members); r=nox

… and 'cargo test', etc. Include Servo and its unit tests, but not Stylo because that would try to compile the style crate with incompatible feature flags: https://github.com/rust-lang/cargo/issues/4463

`workspace.default-members` was added in https://github.com/rust-lang/cargo/pull/4743. Older Cargo versions ignore it.

Source-Repo: https://github.com/servo/servo
Source-Revision: df68eea3f21cc3bbf24d5bbb66be42c4e3a9e427

--HG--
rename : servo/tests/unit/stylo/Cargo.toml => servo/ports/geckolib/tests/Cargo.toml
rename : servo/tests/unit/stylo/build.rs => servo/ports/geckolib/tests/build.rs
rename : servo/tests/unit/stylo/lib.rs => servo/ports/geckolib/tests/lib.rs
rename : servo/tests/unit/stylo/servo_function_signatures.rs => servo/ports/geckolib/tests/servo_function_signatures.rs
rename : servo/tests/unit/stylo/size_of.rs => servo/ports/geckolib/tests/size_of.rs
rename : servo/tests/unit/stylo/specified_values.rs => servo/ports/geckolib/tests/specified_values.rs
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 0939a7049dc771e9d1b4f45f6e3ade2866266fa4