ipatests: test_ipahealthcheck: fix units
df uses 1024 bytes as its default display value, but this can be
tweaked by environment variables or a CLI knob.
Force the output unit to 1024 bytes using the CLI and parse it
accordingly.
Fixes: https://pagure.io/freeipa/issue/8674
Signed-off-by: François Cami <fcami@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>