Fix behavior of connection_pending event

It is now really only raised when Bro is terminating. Also adds a
test-case that raises the event.

testing/btest/scripts/base/protocols/tcp/pending.bro

@@ -0,0 +1,7 @@
+# @TEST-EXEC: bro -C -r $TRACES/tls/chrome-34-google.trace %INPUT
+# @TEST-EXEC: btest-diff .stdout
+
+event connection_pending(c: connection)
+	{
+	print current_time(), "Connection pending", c$id, c$history;
+	}