[lmi] wx_test_default_update.cpp

[Greg Chicares]

I've rewritten the comments in this file, but the code already
does exactly what they say.

I have one concern, though. "MEC avoidance" might not be the
second item on the first tab. For example, it is not so in
'skin_group_carveout.xrc'; we do test distributions that use
that skin, in which case I suspect the test won't succeed.
And of course we might change the layout of other skins in
the future. Is it really onerous to search for this control's
location across all tabs?