tests: Cope with io_method in TEMP_CONFIG in test_aio
If io_method is set in TEMP_CONFIG the test added in
93bc3d75d8e fails,
because it assumes the io_method specified at initdb is actually used.
Fix that by appending the io_method again, after initdb (and thus after
TEMP_CONFIG has been added by Cluster.pm).
Per buildfarm animal bumblebee
Discussion: https://postgr.es/m/zh5u22wbpcyfw2ddl3lsvmsxf4yvsrvgxqwwmfjddc4c2khsgp@gfysyjsaelr5