ifconfig/t_capabilities: s/if_capabilities/if_capenable/ in message

ifconfig/t_capabilities: Skip unless run_unsafe is configured to yes

The test modifies if_capabilities for all available interfaces.
This is not a behavior we expect for normal ATF runs.

Similar tests modifying living network configurations are already
skipped by default. This is the last one remained for ifconfig(8).

Also, I'm not sure whether this is a test for ifconfig(8).

XXX
Pullup to netbsd-10 ASAP. No other branches are affected.

branches: 1.1.6;
Add a basic test for enabling/disabling network interface capabilities.