Add regression test for ALTER TABLE / ADD UNIQUE

OK,

I've attached a patch that adds some regression tests for alter table / add
unique.

One thing that I don't understand tho, is why when adding a unique
constraint, the notice you receive refers to ALTER TABLE/UNIQUE, where in my
code it has ALTER TABLE/ADD UNIQUE - I don't understand...

I've only had the chance to test this test suite on my own machine, so you
may wish to test it elsewhere before committing it.

It includes tests for multi-column indices and indices over oid columns.

Chris