`ExperimentFakes.store()` now returns a store with a different path every
call. This prevents subsequent tests from accidentally using the same
store. Because of this change, we no longer need to delete the on-disk
store in the `assertEmptyStore()` utility, but we still should call it
at the end of every test because enrollments can set prefs which could
cause subsequent tests to fail.
The `browser_contentrelevancy_nimbus.js` test has been updated because it
was not doing any asserts outside of the one buried in FileTestUtils
for creating a temp file, which now only happens when you call
`ExperimentFakes.store()`
Differential Revision: https://phabricator.services.mozilla.com/D242797
f175e9c545