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/net/Cargo.toml

@@ -51,7 +51,7 @@ webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}
 ws = { version = "0.7", features = ["ssl"] }
 
 [dev-dependencies]
-embedder_traits = { path = "../embedder_traits", features = ["tests"] }
+std_test_override = { path = "../std_test_override" }
 
 [[test]]
 name = "main"