feat(dev-infra): add shared testing utilities folder with git mock (#38656)

Adds a new folder to dev-infra where shared testing utilities
could be placed in. This commit already adds initial testing
utilities for dealing with the `GitClient` and SemVer versions.

The `GitClient` in the testing utilities simulates actual Git
behavior in a virtual manner. It's not complete at all, but can
be extended based on our needs. The currently implemented commands
are the most basic ones that we'd need for our release tooling.

PR Close #38656

dev-infra/utils/testing/index.ts

@@ -0,0 +1,11 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export * from './virtual-git-client';
+export * from './virtual-git-matchers';
+export * from './semver-matchers';