testing: Reduce testing runtime by reusing SSH sessions.

By using the ControlMaster and ControlPath options, existing SSH sessions
are reused (a session to each host is opened at the beginning of a test
run).  Also, RC4 is used as encryption algorithm to further improve the
performance.  These changes reduce the runtime by over 20%.

testing/ssh_config

@@ -0,0 +1,10 @@
+Host *
+	# debian default
+	SendEnv LANG LC_*
+	HashKnownHosts yes
+	GSSAPIAuthentication yes
+	# faster encryption
+	Ciphers arcfour
+	# share multiple sessions
+	ControlMaster auto
+	ControlPath /tmp/ssh-uml-%r@%h:%p