Exclude text rendering classes from Tcl smoke tests.

The MatplotlibMathText utilities New() method performs a runtime
check for a valid python environment that has matplotlib installed.
If not found, the New() method returns NULL, which is causing a segfault
on these classes, since the class is not meant to be used directly, but
through the vtkTextRenderer layer.

Change-Id: Ie6a831b1a245969033538f9f3b8013702baeaecc
Merge topic 'fix-tcl-matplotlib-failures' into master

8a40614e Exclude text rendering classes from Tcl smoke tests.