tests/lib/functional_test: Unregister commands on module unload

tests/lib/functional_test: Unregister commands on module unload

When the functional_test module is loaded, both the functional_test and
all_functional_test commands are registered but only the all_functional_test
command is being unregistered since it was the last to set the cmd variable
that gets unregistered when the module is unloaded. To unregister both
commands, we need to create an additional grub_extcmd_t variable.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>