reduce timing sensitivity in SocketConnectorBaseTest

testConnectionFails was expecting the connector thread to exit after
being interrupted, with latency no greater than the configured delay
between connection attempts.  However, the call to
SocketFactory.createSocket may not return immediately on interrupt.  

Now, we allow 4 * DELAY for the thread to exit. This commit should allow
more than enough tolerance, but (as it generally has no effect on the
run duration of the test) we could increase the allowed delay even
further if it proves too sensitive to timing variations on the build
system.
Merge branch 'master' of git@github.com:qos-ch/logback.git