Move rtImageTest.py to vtkmodules.test, and delete vtkTclTest2Py

The vtkTclTest2Py directory contained scripts for converting Tcl test scripts to Python and running them as regression tests. The only thing in the directory that was still important was rtImageTest.py, the test driver. It has been moved into vtkmodules.test.rtImageTest, where it can be executed even from an installed VTK.

A few of the tests were using utility modules from vtkTclTest2Py, and these tests had to be modified. Note that the new test driver does not set VTK_DATA_ROOT as a global variable, as the test scripts should set this themselves so that they can be run without the test driver.