t/lei-refresh-mail-sync: speed up test on FreeBSD 12
And improve reliability while we're at it. It seems closing a
TCP listen socket on FreeBSD 12.2 doesn't cause connect()-ing
clients to fail. This happens regardless of whether a socket is
IPv4 or IPv6
This non-failure was causing tests to timeout slowly on the
client side instead of failing immediately. We now fork a new
process which does nothing but accept() + shutdown() to emulate
a dead server.
Reliability improves on all OSes since there's never a point in
time when another process can bind the socket.