full model name confuses zaza when test-directory is used

[marosg42]

When full model name (controller:model) and test-directory are specified, zaza pretends there are no tests

$ functest-test -m foundations-maas:ceph-standalone --test-directory tests/ 
2023-01-25 10:45:31 [INFO] Events: {}
Metadata: {}

The same without controller name

$ functest-test -m ceph-standalone --test-directory tests/ 
2023-01-25 10:47:18 [INFO] ## Running Test zaza.openstack.charm_tests.ceph.mon.tests.CinderCephMonTest ##
...

But when -t is used to specify a test, controller:model is fine

$ functest-test -m foundations-maas:ceph-standalone -t zaza.openstack.charm_tests.ceph.tests.CephRGWTest
2023-01-25 10:49:28 [INFO] ## Running Test zaza.openstack.charm_tests.ceph.tests.CephRGWTest ##
...

When not using --test-directory at all -t works fine with and without controller