Fix bug with mutation expectations
Fix bug with mutation expectations

If you run a unit test that expects some mutations:

```
withMutations([{ tagName: 'button', max: 1}], ...)
```

This test would fail if a button mutation was seen >1 times. However any
other mutations would be ignored. If your code also mutated some `p` tag
500 times, this would not be seen as a test failure.

This CL updates the mutation checker to find any mutations that were not
matched to a provided expectation from the developer, and errors for
them. We ignore #comment nodes because Lit-Html uses them to track DOM,
but it'll fail on any other DOM.

This change has been tested on the data-grid CL, which is WIP, where I
first discovered the problem.

Bug: 1129881
Change-Id: I61af7e92f6793f97d05b910504b3539274ea5444
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2436816
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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