ENH: Coverage for MultiThreshold.

Add a Unit Test for vtkMultiThreshold. This filter was not being
tested at all. During the validaiton of test results an error was
detected in the XOR boolean operation. The error is also corrected
in the patch.

Change-Id: I4b8b53d602f0af48b3e93c1504220006e94418e4
Merge topic 'Coverage_MultiThreshold' into master

f3311d5c ENH: Coverage for MultiThreshold.