First attempt towards automated tests

[riebl]

Let me briefly outline the setup:

• tests/CMakeLists.txt defines the OmnetppCmakeTests project
• the testing project initialises several (currently only one for demonstration) test suites via add_test
• add_test employs RunTestSuite.cmake to run a test suite, which is an CMake project itself, as a separate process
• tests/find-omnetpp/CMakeLists.txt is an example of a test suite that exercises FindOmnetPP.cmake via a find_package call
• some test suite helper macros allow to check if FindOmnetPP.cmake found the expected OMNeT++ installation
• the expected values are passed as environment variables (see Dockerfile)

Any feedback is highly appreciated.