BUG: don't add keyword arguments to unittest.TestCase constructor

The superclass generates an error when the subclass has keyword
arguments it doesn't know.

Instead we enforce the default value when used.

For a subclass or a user or the class to change the value of this
instance variable they can just set it directly, as in:

self.messageDelay = 50

which can be done from anywhere in the code.

See discussion here:
http://massmail.spl.harvard.edu/public-archives/slicer-devel/2015/018624.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24558 3bd1e089-480b-0410-8dfb-8563597acbee