Auto merge of #21884 - servo:unit-rebuild, r=emilio

Make `mach test-unit` not recompile components after `mach build`

Previously, the `tests` feature flag of the `embedder_traits` crate caused it and every crate recursively depending on it to be built twice.

This feature flag was used to provide a specific set of "resources" when running tests. Instead, this commits overrides the `main()` function of the test harness to change resources at runtime before running any test.

This is done by adding a dependency that has `name = "test"` in its `[lib]` section of `Cargo.toml`. This overrides the crate found by `extern crate test;` in code generated by `rustc --test`.

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