syslogd: Split up basic test case

The basic case previously tested if messages are correctly logged over
UNIX, INET, and INET6 transport. This single case can be split up into
three separate cases to decrease code complexity and offer more granular
results.

Both INET and INET6 cases will be skipped if the kernel does not
support the corresponding transport.

Reviewed by: zlei, markj
Differential Revision: https://reviews.freebsd.org/D47650