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/std_test_override/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "std_test_override"
+version = "0.0.1"
+authors = ["The Servo Project Developers"]
+license = "MPL-2.0"
+publish = false
+
+[lib]
+name = "test"
+path = "lib.rs"
+
+[dependencies]
+embedder_traits = { path = "../embedder_traits" }

components/std_test_override/lib.rs

@@ -0,0 +1,15 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#![feature(test)]
+
+extern crate embedder_traits;
+extern crate test;
+
+pub use test::*;
+
+pub fn test_main_static(tests: &[&TestDescAndFn]) {
+    embedder_traits::resources::set_for_tests();
+    test::test_main_static(tests);
+}