Small PR to fix up some issues where the functionality in #903 was causing some warning messages. In particular, the following snipped was causing a warning (from matlab_test.py):

siso_tf2 = tf([1, 1], [1, 2, 3, 1])
siso_ss2 = ss(siso_tf2)
[gm, pm, gmc, pmc] = margin(siso_ss2 * siso_ss2 * 2)

The reason for the warning was that siso_ss2 had the name 'sys[0]$converted' and when you create an interconnected system with duplicated non-generic names it gives you a warning (since things like change the parameters of one of the systems would affect the other.

To fix this, converted LinearIOSystems with "generic" names (sys[0]) do not get '$converted' added to the end. Instead they just get a new generic system name.

That change meant that some of the previous unit tests needed to use an explicitly named system, which is what the the changes in the unit tests are about.