Code review comment for ~peter-sabaini/charm-grafana:test-fixes-race-cleanup

The focal-snap bundle failed on the upgrade tests oddly:

https://pastebin.canonical.com/p/zQmBpDFyW2/

bionic-snap had no such trouble, and the rest of tests run.

2021-01-29 21:12:10 [INFO] ======================================================================
2021-01-29 21:12:10 [INFO] FAIL: test_50_snap_upgrade (tests.test_grafana.SnappedGrafanaTest)
2021-01-29 21:12:10 [INFO] Test a snap upgrade from 6 to 7.
2021-01-29 21:12:10 [INFO] ----------------------------------------------------------------------
2021-01-29 21:12:10 [INFO] Traceback (most recent call last):
2021-01-29 21:12:10 [INFO] File "./tests/test_grafana.py", line 310, in test_50_snap_upgrade
2021-01-29 21:12:10 [INFO] "PACKAGE UPGRADE REQUIRES MANUAL INTERVENTION" in status_message
2021-01-29 21:12:10 [INFO] AssertionError: False is not true
2021-01-29 21:12:10 [INFO] ======================================================================
2021-01-29 21:12:10 [INFO] FAIL: test_51_snap_downgrade_block (tests.test_grafana.SnappedGrafanaTest)
2021-01-29 21:12:10 [INFO] Test that after the previous upgrade, downgrades are blocked.
2021-01-29 21:12:10 [INFO] ----------------------------------------------------------------------
2021-01-29 21:12:10 [INFO] Traceback (most recent call last):
2021-01-29 21:12:10 [INFO] File "./tests/test_grafana.py", line 324, in test_51_snap_downgrade_block
2021-01-29 21:12:10 [INFO] self.assertTrue("PACKAGE DOWNGRADES ARE NOT SUPPORTED" in status_message)
2021-01-29 21:12:10 [INFO] AssertionError: False is not true
2021-01-29 21:12:10 [INFO] ----------------------------------------------------------------------