tipc_test: allow connecting to arbitrary services

Add an option to specify a service name. Currently only the "connect"
test supports this, and will try to connect to the named service
instead of the echo and datasink services.
Also fix the built-in help text, as it was missing the "burst" option,
and didn't mention that the "connect" test connects to both the echo
and datasink services.

Bug: 200034376
Test: run

Change-Id: I3903f94c16320f8e07ac4bfb69e3bb45329138c0