Add more tests on event triggers

Here are some more tests for event triggers, especially about new features
introduced recently in 296f3a6053844089bc533630fffafaba8f016384.

Check that COMMENT, GRANT and REVOKE do trigger some events.

The patch also check some more error cases: SQL function, functions with
declared arguments, arguments passed to the trigger, event trigger on
global objects (ROLE, TABLESPACE, DATABASE).

I have left out "SECURITY LABEL" which would require a special security
label provider. Note that they do not seem to be tested at all anyway:-(
The "security_label.sql" really tests that all fails when no plugin is
available. If some actual tests are added to security label some day, I
think that event triggers about them should be tested there.

--
Fabien.