ENH: Update vtkMRMLColorTableNodeTest1 to test against expected output

This commit updates the test to check that color table entry
can be written and read from a color table file.

Some background into ...

Commit r20729 attempted to implement a test checking that the color table
entry could be written to a scene and read from the scene. The issue was
that the test was only displaying value and was not testing against
expected value.
Then, in commit r22344, the color mrml nodes have been updated to
to not write anymore into the mrml scene.

The test wasn't reporting that no color could be read but was still
passing, this commit updates the test and check for expected values instead
of just displaying them.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23448 3bd1e089-480b-0410-8dfb-8563597acbee