These Python tests have been refactored.

This has been done by removing the "tcl-isms" and making
 the code more readable. In some cases redundant code
 has been removed (often a result of copy/paste operations).
 The code has also been checked to ensure each line is 80
 characters or less.

The code can now be run using vtkpython or by using the
 appropriate Python environment.

Originally these were TCL scripts that were converted using
  vtkTclTestToPy.py and they they needed to be run through the
  rtImageTest.py driver which defines a few needed
  functions/variables for their operation. To this end
  backdrop.py has been added where it is needed.

Change-Id: I50d6505628c140ea4551f74f7f3e85df907567e1
Signed-off-by: Andrew Maclean <andrew.amaclean@gmail.com>
Merge topic 'RefactorPythonTests' into master

3810c5a6 These Python tests have been refactored.