pgsql: Fix buildfarm error from commit 5c31669058. - Mailing list pgsql-committers

Fix buildfarm error from commit 5c31669058.

Skip test when not using unix domain sockets.

Discussion: https://postgr.es/m/CALDaNm29-8OozsBWo9H6DN_Tb_3yA1QjRJput-KhaN8ncDJtJA@mail.gmail.com
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5b5318c387451e3eb89eddb4574e57a61297102f

Modified Files
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src/test/subscription/t/027_nosuperuser.pl | 151 ++++++++++++++++-------------
1 file changed, 81 insertions(+), 70 deletions(-)