Add two tests asserting watch cache behavior (with and without the plugin).
The lack of extra asset caching causes webpack to emit css files on every change (and report build taking place with noop JS changes).

Add support for skipping watch test steps when no build is expected.

Now that "extract-text-webpack-plugin" correctly adds asset hashes to compilation hash, this can be removed.

I'm actually not sure whether it makes sense to keep it (not aware of any other plugins affected by this),
But I'd remove it at least for now, because of how extract-text-webpack-plugin works today.
Child compilation is only added if css changes, which breaks test case watchCases/plugins/extract-text-plugin step 3 (no-op JS change) on Windows and Linux.
It works on macOS though, but only as a result of another bug: watchpack reports all files as changed on that platform.

Excuse: beautify-lint does not match file patterns with forward slash on Windows