build(unit_tests): custom CMake helper for adding unit tests

[dssgabriel]

This PR adds a custom CMake helper function for easily adding new unit tests.

I made it as flexible as possible, so it should cover all cases we may encounter for existing and future unit tests. Also added some custom gersemi definitions so our CMake formatter can parse and format these function calls properly.

Applied this new function to all our existing unit tests and renamed them using the following format: $kind.$module.$name, where:

• kind: {smoke,test}
• module: {core,mpi,nccl}

Example call for the pending PR #191 that adds the "core" broadcast unit test:

kc_add_unit_test(
  test.core.broadcast                     # Full test name
  CORE                                    # Test module
  NUM_PES 2                               # Number of Processing Elements (PEs) to run the test with
  FILES test_main.cpp test_broadcast.cpp  # Files defining the test
)