TEST: implement CMake function imstk_add_test

Allows to create tests in iMSTK using google test and ctest.
Ctests are created by test files, and not by gtest functions.

Requirements:
- Requires a 'Testing' subdirectory with cpp files for each
  class to test.
- Google Test and Google Mock should be used to create unit
  tests, see the documentation below:
  https://github.com/google/googletest/blob/master/googletest/docs/Documentation.md
  https://github.com/google/googletest/blob/master/googlemock/docs/Documentation.md
- Test files should be constructed as described under\
  `imstk_add_test()` declaration