Add wait_for_keymgr_done() util function to tests

Add wait_for_keymgr_done() util function to tests

The kasp test cases assume that keymgr operations on the zone under test
have been completed before the test is executed. These are typically
quite fast, but the logs need to be explicitly checked for the messages,
otherwise there's a possibility of race conditions causing the
kasp/rollover tests to become unstable.

Call the wait function in all the kasp/rollover tests where it is
expected (which is generally in each test, unless we're dealing with
unsigned zones).