Remove collate.linux.utf8.sql's assumptions about ".utf8" in locale names.

Remove collate.linux.utf8.sql's assumptions about ".utf8" in locale names.

Tweak the test so that it does not depend on the platform using ".utf8" as
the extension signifying that a locale uses UTF8 encoding.  For the most
part this just requires using the abbreviated collation names "en_US" etc,
though I had to work a bit harder on the collation creation tests.

This opens the door to using the test on platforms that spell locales
differently, for example ".utf-8" or ".UTF-8".  Also, the test is now
somewhat useful with server encodings other than UTF8; though depending on
which encoding is selected, different subsets of it will fail for lack of
character set support.