ENH: Add support for absolute test paths in SLICER_ADD_PYTHON_TEST macro

The macro did not allow for full paths describing the test scripts and
always defaulted to CMAKE_CURRENT_SOURCE_DIR.

This was changed so the macro automatically add the current source
directory to the script path ONLY if the script path isn't absolute. We can
now do something like:

SLICER_ADD_PYTHON_TEST(
  SCRIPT /absolute/path/to/script.py
)
or
SLICER_ADD_PYTHON_TEST(
  SCRIPT ../script.py
)