test(docs-infra): destroy all `ScrollService` instances after each test (#33937)
test(docs-infra): destroy all `ScrollService` instances after each test (#33937)

`ScrollService` subscribes to global `window` events and mutates global
state in the listener (e.g. read/write values from/to `sessionStorage`).
Therefore, we need to always call its `ngOnDestroy()` method to
unsubscribe from these events after each test.

In f69c6e204, a new testcase was introduced that was not destroyed. As a
result, random failures started to randomly happen in other, unrelated
tests ([example CI failure][1]).

This commit fixes this by ensuring all `ScrollService` instances are
destroyed after each tests (provided that they are created with the
`createScrollService()` helper).

[1]: https://circleci.com/gh/angular/angular/533298

PR Close #33937

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