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

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).

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/72a3ebf235b52fdb715214027026f66a0ad7da4d

Modified Files
--------------
src/test/regress/expected/regex.linux.utf8.out | 11 +++++++++--
src/test/regress/expected/regex.linux.utf8_1.out | 11 +++++++++++
src/test/regress/parallel_schedule | 2 +-
src/test/regress/sql/regex.linux.utf8.sql | 12 ++++++++++--
4 files changed, 31 insertions(+), 5 deletions(-)