Tests: Make fake_notifier per-instance

Tests: Make fake_notifier per-instance

Tests have been failing because multiple threads modify the
same global NOTIFICATIONS list.  Move the notification storage
to be per-test instance, which is what the tests assume.

This has been broken for a while, but made more obvious by
recent changes to parallelize tests.

Also remove all the extra reset calls we added to work around
this problem over time.

Co-authored-by: Gorka Eguileor <geguileo@redhat.com>
Change-Id: Ib5a67c097ef548dc346751ad51b61fdceaf8e3a8
Partial-Bug: #1259463
Closes-Bug: #1412513