test: Fix gen_conf test cases

Test case `test_gen_conf_vlan_parent_ref_by_mac` will fail on Ubuntu
hosted CentOS stream 9 container. Investigation is on going at

https://issues.redhat.com/browse/NMT-1885

As workaround, explicitly load NM key file via `nmcli load <file_path>`
instead of `nmcli c reload` fix it.

Signed-off-by: Gris Ge <fge@redhat.com>