We currently have two long-standing issues related to how `TestBed` tests are torn down:
1. The dynamically-created test module isn't going to be destroyed, preventing the `ngOnDestroy` hooks on providers from running and keeping the component `style` nodes in the DOM.
2. The test root elements aren't going to be removed from the DOM. Instead, they will be removed whenever another test component is created.
By themselves, these issues are easy to resolve, but given how long they've been around, there are a lot of unit tests out there that depend on the broken behavior.
These changes address the issues by introducing APIs that allow users to opt into the correct test teardown behavior either at the application level via `TestBed.initTestEnvironment` or the test suite level via `TestBed.configureTestingModule`.
At the moment, the new teardown behavior is opt-in, but the idea is that we'll eventually make it opt-out before removing the configuration altogether.
Fixes#18831.
PR Close#42566
