ping_test: Fix tests ping_46 and ping6_46

If no IPv4-host, IPv4-mcast-group or IPv6-host is passed, it will
display the usage. The tests are passing because they are just checking
that the exit code is 1.

Fix the tests by checking the appropriate output message.

While here, change the description to match the output and add the
missing requirements.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37250