persistent-test: Beef up 'Migration : is idempotent' test.

The original "is idempotent" test did not include any Unique constraints.
Adding such a constraint make this test fail.

Also worth running an "is idempotent" test more than once :-).
persistent-postgresql: Fix #204.

The SELECT statement which read the constraints for the table had a
component:

     AND c.ordinal_position=1

which meant that when reading the constraint, only the first constraint
was read so that when it compared what was read with the schema the
migration code decided to DROP the existing constraint and then ADD the
correct constraint.

The fix is simply to drop the above line from the SELECT.
Merge remote-tracking branch 'origin/master'