tests/unit/test-qmp-event: Simplify event emission check

tests/unit/test-qmp-event: Simplify event emission check

The generated qapi_event_send_FOO() call an event emitter function.
It's test_qapi_event_emit() in this test.  It compares the actual
event to the expected event, and sets a flag to record it was called.
The test functions set expected data and clear the flag before calling
qapi_event_send_FOO(), and check the flag afterwards.

Make test_qapi_event_emit() consume expected data, and the test
functions check it was consumed.  Delete the flag.  This is simpler.
It also catches extraneous calls of test_qapi_event_emit().  Catching
that is not worthwhile, but since the cost is negative...

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20231122072456.2518816-3-armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>