tests: Fix failing ClearPartTestCase with latest blivet (#test)
Blivet now uses parted_partition.number for sorting partitions so
we need to set it for the Mock in test_should_clear.
See also https://github.com/storaged-project/blivet/pull/960
(cherry picked from commit 5bbdd202ae092f8ddd50eda726b3b9f61465e21c)