Move LibCompress unit tests to LibCompress/Tests directory and register
them with CMake's add_test. This allows us to run these tests with
ninja test instead of running a separate executable.
Also split the existing tests in 3 test files that better follow the
source code structure (inspired by AK tests).
Very incompressible data could sometimes produce no backreferences
which would result in no distance huffman code being created (as it
was not needed), so VERIFY the code exists only if it is actually
needed for writing the stream.
