macOS is shipped with a very old version of the bash (3.2), which
is currently not suitable for running the iotests anymore (e.g.
it is missing support for "readarray" which is used in the file
tests/qemu-iotests/common.filter). Add a check to skip the iotests
in this case - if someone still wants to run the iotests on macOS,
they can install a newer version from homebrew, for example.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200918153514.330705-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>