Fix UUID sortability tests in base32hex encoding.

Fix UUID sortability tests in base32hex encoding.

Commit 497c1170cb1 added base32hex encoding support, but its
regression test for UUIDs failed on buildfarm members hippopotamus and
jay using natural language locales (such as cs_CZ). This happened
because those collations may sort characters differently, which breaks
the strict byte-wise lexicographical ordering expected by base32hex
encoding.

This commit fixes the regression tests by explicitly using the C
collation.

Per buildfarm members hippopotamus and jay.

Analyzed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/682417.1774482047@sss.pgh.pa.us