Hopefully-portable regression tests for CREATE/ALTER/DROP COLLATION.

The collate.linux.utf8 test covers some of the same territory, but
isn't portable and so probably does not get run often, or on
non-Linux platforms.  If this approach turns out to be sufficiently
portable, we may want to look at trimming the redundant tests out
of that file to avoid duplication.

Robins Tharakan, reviewed by Michael Paquier and Fabien Coelho,
with further changes and cleanup by me.