Adding a new test for TestSocketCommunicator.

Add a tests for vtkSocketCommunicator. Also added a driver (vtkTestDriver)
that can be used when adding tests that need to run multiple executables.
Currently vtkTestDriver is simple: it simply spawn executables with commandlines
specified.

Change-Id: I03a53c827a99ba02519442f3059f86cd3d9484e5
Merge topic 'add_socket_communicator_test' into master

3e5e0e1a Adding a new test for TestSocketCommunicator.