arcyd/acceptance_tests: exercise config ref more

Exercise conditions 'no config ref', 'config ref, no repo.json' and
'config ref, repo.json only contains {}'.

Test Plan:
$ ./testbed/arcyd/acceptance_tests.sh
Arcyd: check if config ref exists before loading

Only attempt to load from config ref if it exists, this makes the
try..except slightly less broad.

Add phlgit_showref for retrieving local refs.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh