syslogd/tests: Fix flakiness in forwarding tests

[markj]

Summary

syslogd_start() waits for the local log socket to appear before
returning, to ensure that the daemon is ready to handle log messages.
Some tests start two daemons, so by default the socket already exists
when the second daemon is started, so syslogd_start() returns early.
The test subsequently sends a message to this second daemon, which
sometimes isn't ready.

Define a separate log socket for the second daemon. Add a check to
syslogd_start() to help catch this type of bug.

[jlduran]

I've seen that when using ATF, ${PWD} is preferred over $(pwd) (efficiency, I guess).

[markj]

Use ${PWD}