Code review comment for lp:~percona-toolkit-dev/percona-toolkit/fix-1062563-1063912-ptc-pxc-bugs

The PXC code in Cxn.pm doesn't seem to belong there. Let's move it and its tests to separate files, i.e. Percona/XtraDB/Cluster.pm (or maybe /XtraDBCluster.pm, but that looks a little cluttered to me). Then like I did with t/pt-table-checksum/pxc.c, we can plan skip_all at the start if the sandbox isn't running PXC.