Draft: [test] Introduce test suite based on Google Test

Adds simple unit testing to SCDAQ as a subproject through Google Test.

Two simple test files currently exist:

• scdaq_config, with basic coverage for opening, loading and parsing of configuration files.
• scdaq_operation which validates sustained operability in fileDMA contexts, similarly to the runFileTest.sh step in the current CI pipeline.

If GoogleTest is not installed as a system library, the dedicated CMakeLists.txt automatically downloads it in the build directory.

TJ - changing to target feature/cmake-migration to allow for merge review