Add unit test suites with initial successful dummy test (#781978)

Now 'make check' will additionally build and run the test suites in the
./tests directory.  Add initial always successful dummy test suite.
This is done using Automake support for testing.

    Automake Manual, 15 Support for test suites
    https://www.gnu.org/software/automake/manual/automake.html#Tests

./tests/Makefile.am takes some influence from the same file in the
minimal-gtest-autotools template project.
    https://github.com/octol/minimal-gtest-autotools/blob/654848ec0164a80f851fa80b8355099f35c025a2/tests/Makefile.am

Bug 781978 - Add Google Test C++ test framework