Modify mean test

The mean test previously asserted that the mean of a random buffer was 15.625.  The mean op was subsequently changed to return "sum/size" (previously the op had returned "size/sum"). Thus the test needed to be changed to assert that the mean of the buffer is 1/15.625.

An additional simple "sanity check" was also added that works as follows:

1.  Populate all pixels of an image with a constant value of 100.0.
2.  Calculate the mean using the op.
3.  Assert that the mean is equal to the constant value of 100.0.