test: add registry/rpc to the list of unit tests
test: add registry/rpc to the list of unit tests

A new directory registry/rpc should be added to the list of unit tests,
so that ./test can test also unit tests under RPC registry.
Without this change, even compile errors in unit tests cannot be
easily detected, because the ./test ignores the unit tests.

jonboulle/fleet,endocode/fleet,coreos/fleet,jonboulle/fleet,endocode/fleet,coreos/fleet,coreos/fleet,giantswarm/fleet,dalbani/fleet,dongsupark/fleet,jonboulle/fleet,giantswarm/fleet,giantswarm/fleet,endocode/fleet,dalbani/fleet,dongsupark/fleet,dalbani/fleet,dongsupark/fleet