test-certificate-chain: Wait for objects being released by GTask's

As per switching to GTask we are facing glib#1346 in various places, including the certificate tests, as we were checking whether the mock certificates were finalized in the same thread in which we created them.

This may not always happen right now because GTask may release the last reference of it later than we do. So, to force this to happen let's wait until we're owing last reference and only at that point we actually release it, so that we can be sure that the objects are finalized on tear-down.

Fixes: #84 (closed)