Separate Python tests requiring C++ modules into separate testing directory.

[molpopgen]

Several tests use compiled modules, which is irritating for several reasons:

• they are built when the primary module is built.
• they make it impossible for someone to run the Python test suite after installing a shell from PyPi. (The compiled test modules won't be there.)
• probably other things..

I also suspect that the utility of some of these tests may be minimal. Such tests could be removed.

One idea to fix:

• make a new tests_with_cpp directory
• move the tests over to there
• separate the build system out from the main package module into the new directory.
• update the various testing work flows to compile and run them.