mctest.php: nicely align server names

The servers entries being made of an IP address and a port, they would
often have different lenghts which clutter the output indentation.

Previously the output columns would be slightly misaligned:

$ mwscript mctest.php --wiki=enwiki
10.4.1.255:11211 set: 100   time: 0.17192602157593
10.4.1.1:11211 set: 100   time: 0.19056081771851
$

With this patch:

$ mwscript mctest.php --wiki=enwiki
10.4.1.255:11211 set: 100   time: 0.17192602157593
10.4.1.1:11211   set: 100   time: 0.19056081771851
$

Change-Id: I80199a229ed6310686411f08e0344c7d906414ea
Merge "mctest.php: nicely align server names"