Disable the aio-max-nr 20.t test

The 20.t test that tries to ensure that aio-max-nr requests can be allocated is failing on OBS/amd64:

Starting cases/20.p
aio_max_nr: 65536
Creating 65536 ioctx-s with 1 events each...
io_setup(1) failed on iteration 1365.
allocated 1365 of 65536 possible events.
aio_nr is currently at 131040
test cases/20.t completed FAILED.
Completed cases/20.p with 1 -- FAILED.

On arm64 it failed a couple of times and then passed. On armhf it passed at the first try.

However, this is not a regression: the same test failed on v2022dev1 but test harness failures were simply not considered fatal by the packaging.

With the update from Bullseye they are not considered fatal. Let's disable the failing test for now.