Enable C++ unit testing

[thbeu]

This PR

• adds the C++ unitests of #29 for DBFOpen and DBFCreate using the ~Catch2~GTest framework
• moves the CMake test configuration to test.cmake

According to Catch2 it is recommended to have one test binary per library. I'd say, there can be binaries for SHP, DBF and SBN unit tests etc.

Closes #29