fix(ngMock): prevent memory leak due to data attached to `$rootElement`

Starting with 88bb551, `ngMock` will attach the `$injector` to the `$rootElement`, but will never
clean it up, resulting in a memory leak. Since a new `$rootElement` is created for every test,
this leak causes Karma to crash on large test-suites.
The problem was not detected by our internal tests, because we do our own clean-up in
`testabilityPatch.js`.

88bb551 was revert with 1b8590a.
This commit incorporates the changes from 88bb551 and prevents the memory leak, by cleaning up all
data attached to `$rootElement` after each test.

Fixes #14094

Closes #14098