Move rowsecurity event trigger test

The event trigger test for rowsecurity can cause problems for other
tests which are run in parallel with it.  Instead of running that test
in the rowsecurity set, move it to the event_trigger set, which runs
isolated from other tests.

Also reverts 7161b082, which moved rowsecurity into its own test group.
That's no longer necessary, now that the event trigger test is gone from
the rowsecurity set of tests.

Pointed out by Tom.