Re-enable tests using collection

Awhile ago, these tests were intermittently failing on Jenkins.
I never got them to fail on my local machine.
We never quite figured out why they were failing & the failing
was inconsistent: most of the time, tests ran just fine;
other times, however, they just failed.

We've made lots of changes since then. We've made some changes
to our storage as well. Let's try re-enabling them again and
see what happens. They still pass on my local machine.

We can always revert this patch if we start seeing them fail
again (or actually try to figure out exactly why they're failing)

Change-Id: I8cfc92a46e5b79570b37805a75a29a3bf3c59432
Cleanup LinksTableTest

It was doing a lot of things that are already provided in
PostRevisionTestCase, like generating PostRevisions objects.

Change-Id: I826fe37afb090a2fc04f91508f6d02ed940e49b2