This commit updates the implementations of `autoDetectChanges` to be
shared between the zone-based and zoneless fixtures. This now allows
`autoDetect` to be turned off for zoneless fixtures after it was
previously on because the host view is no longer directly attached to
`ApplicationRef`.

This commit moves the ngZone onError subscription to the base fixture
implementation. While this subscription isn't necessary for zoneless, it does
no harm because the observable never emits.

This commit removes the abstract base class and two separate
implementations of `ComponentFixture` for zone vs zoneless. Now that the
behaviors have gotten close enough to the same, the diverged concrete
implementations serve less value. Instead, the different behaviors can
be easily handled in if/else branches. The difference is now limited to
the default for `autoDetect` and how `detectChanges` functions.

Disabling `checkNoChanges` in `ComponentFixture.detectChanges` was an
error for the zoneless fixture since it was not yet working. This now
allows checkNoChanges to be disabled. This option isn't really
used/shouldn't be used by anyone except the FW so marked as a refactor.

…eless

When disabling autodetect (not recommeneded) with zoneless,
`fixture.detectChanges` would previously not refresh the fixture's component.