Use a short socket directory path in pg_upgrade testing.

Several buildfarm members are failing the pg_upgrade test in
REL_15_STABLE, though the identical test is fine in HEAD.
On thorntail it's possible to see that the problem is an
overlength socket path name, and I bet the same is true
on the others.

The normally-started postmasters used in the test are already
set up with short socket directory paths, but we neglected to
tell pg_upgrade itself to do likewise when starting child
postmasters, and indeed it seems to be explicitly selecting
the test directory instead.

Back-patch to v15 where the current test script was introduced.
(The previous script might have the same issue, because I don't
see any use of -s/--socketdir in it either; but we've had no
complaints, so leave it alone for now.)

Discussion: https://postgr.es/m/1410025.1656890531@sss.pgh.pa.us

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