Disable 031_recovery_conflict.pl in 15 and 16.

Disable 031_recovery_conflict.pl in 15 and 16.

This test fails due to known bugs in the test and the server.  Those
will be fixed in master shortly and possibly back-patched a bit later,
but in the meantime it is unhelpful for package maintainers if the tests
randomly fail, and it's not a good time to make complex changes in 16.

This had already been done for older branches prior to 15's release.
Now we're about to release 16, and Debian's test builds are regularly
failing on one architecture, so let's do the same for 15 and 16.

Reported-by: Christoph Berg <myon@debian.org>
Reported-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CALj2ACVr8au2J_9D88UfRCi0JdWhyQDDxAcSVav0B0irx9nXEg%40mail.gmail.com