Copy the test_data/ RLS tests into a writable directory.

See rust-lang-nursery/rls#966 for details.

src/bootstrap/test.rs

@@ -290,6 +290,13 @@ fn run(self, builder: &Builder) {
                                                  "src/tools/rls",
                                                  SourceType::Submodule);
 
+        // Copy `src/tools/rls/test_data` to a writable drive.
+        let test_workspace_path = builder.out.join("rls-test-data");
+        let test_data_path = test_workspace_path.join("test_data");
+        builder.create_dir(&test_data_path);
+        builder.cp_r(&builder.src.join("src/tools/rls/test_data"), &test_data_path);
+        cargo.env("RLS_TEST_WORKSPACE_DIR", test_workspace_path);
+
         builder.add_rustc_lib_path(compiler, &mut cargo);
 
         if try_run(builder, &mut cargo) {