Retry client connections in selftest

Replace a hard-coded delay after process start with a dynamic client
connection retry, which attempts to connect up to five times with
the first being immediate while subsequent connections observe an
exponential backoff over a three-second period before giving up.
This improves both speed and stability of test runs.

Also combine the ConnectionResetError and EOFError conditions which
could be raised later once the socket is in use, as the end result
of either is effectively the same.