Add support for "noop" changes in watch tests

[yuriyostapenko]

What kind of change does this PR introduce?
Adds watch test for "noop" change and support for skipping steps in watch tests

Did you add tests for your changes?
It is only tests

If relevant, link to documentation update:

Summary
Allows to skip test steps that are not supposed to trigger compilation changes

Does this PR introduce a breaking change?
No

Other information
Extracted from #3806

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[sokra]

With this change, broken test cases can be "skipped" if some change incorrectly doesn't trigger a rebuild. This can hide failures.

Make sure that skipping is opt-in. Maybe with a file .skip in the directory.

[webpack-bot]

For maintainers only:

• This need to be documented (issue in webpack/webpack.js.org will be filed when merged)