BUG: Ensure scripted module are deleted when exiting Slicer

Similarly to what was done in r24230 (ENH: Ensure slicer.(modules|moduleNames)
is updated when a module is unloaded), this commit also ensures that
"slicer.selfTests" dictionary is updated when a module is unloaded.

This change has been tested by adding a print statement in the "del"
function of both the script module and module widget python class for the
DataProbe, Editor and Endoscopy module.

After this change, it could only confirm that the "del" function for
the module class was called for all three modules. More work is required
to also ensure the scripted module widget class is deleted.

Instead of explicitly updating the selfTests dict, an other possible
approach would be to use a WeakValueDictionary [1]

[1] https://docs.python.org/2/library/weakref.html#weakref.WeakValueDictionary

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24239 3bd1e089-480b-0410-8dfb-8563597acbee