Testing: Verify mutation triggers in worker-thread

**Suggestion**: Element tests in src/test/ should verify that `mutator.ts` is invoked with the correct params. Refactoring `mutator.ts` from module-level vars into a class would help mockability in unit tests.

For example:
```js
test('HTMLInputElement.value', t => {
const {element, mutator} = t.context;
element.value = 123;
expect(mutator.mutate).calledWithMatch({type: PROPERTIES, value: 123});
});
```

Start by reading src/test/ and mutator.ts, then trace how an element mutation reaches the worker-thread mutator. Use the HTMLInputElement.value example as the first test case and verify that mutate receives the expected type and value parameters. Done means the relevant element tests cover these calls and the mutator is mockable as described.

Written by the indexing model from the issue text.