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`.

components/bluetooth/Cargo.toml

@@ -19,6 +19,3 @@ log = "0.4"
 servo_config = {path = "../config"}
 servo_rand = {path = "../rand"}
 uuid = {version = "0.6", features = ["v4"]}
-
-[dev-dependencies]
-embedder_traits = { path = "../embedder_traits", features = ["tests"]}