Run UTF8-requiring collation tests by default

Run UTF8-requiring collation tests by default

The tests collate.icu.utf8 and collate.linux.utf8 were previously only
run when explicitly selected via EXTRA_TESTS.  They require a UTF8
database, because the error messages in the expected files refer to
that, and they use some non-ASCII characters in the tests.  Since
users can select any locale and encoding for the regression test run,
it was not possible to include these tests automatically.

To fix, use psql's \if facility to check various prerequisites such as
platform and the server encoding and quit the tests at the very
beginning if the configuration is not adequate.  We then need to
maintain alternative expected files for these tests, but they are very
tiny and never need to change after this.

These two tests are now run automatically as part of the regression
tests.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/052295c2-a2e1-9a21-bd36-8fbff8686cf3%402ndquadrant.com