fix: Clear semantic instances for TV3 migration tests
part of request #37944 Flaky TV3 migration tests
No functional change.
$ make tests-db SEED=
1715949306
The command above should succeed.
Why?
Title, status and contributor are being asserted on, but they have
caches on tracker ID. If a previous test created a tracker with the same
ID, and loaded its title (status, etc.), then the TV3 tests will re-load
the same title and fail the assertions.
Change-Id: I14fe9f190dbc5de023986fef47886377d11fa4e5