add (minimal) UPDATE regression test

This patch adds a new regression test for the UPDATE command. Right
now all that is tested is Rod Taylor's recent addition to allow
this syntax:

    UPDATE ... SET <col> = DEFAULT;

If anyone else would like to add more UPDATE tests, go ahead --
I just wanted to write a test for the above functionality, and
couldn't see an existing test that it would be appropriate
to add to.

(Bruce, this patch adds 2 new files, src/test/regress/sql/update.sql
and src/test/regress/expected/update.out)

-Neil