Enable routine running of regex.linux.utf8 regression test.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 72a3ebf235b52fdb715214027026f66a0ad7da4d
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2022-01-05T22:31:54Z
Releases: 15.0
Enable routine running of regex.linux.utf8 regression test.

Up to now this has just sat there as a test you could invoke via
EXTRA_TESTS, which of course nobody does.  I'm feeling encouraged
because c2e8bd275 hasn't yet broke anything, so let's try making this
run with a suitable guard condition (similar to collate.linux.utf8).

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