Add environment-agnostic scripts for running ctests and pytests

[trxcllnt]

This PR adds environment-agnostic run_*_{ctests,pytests}.sh scripts, and updates test_*_{cpp,python}.sh to call them.

The test_*_{cpp,python}.sh scripts assume they're running in our CI environment, and they do more than just run the tests.

This PR allows devs and downstream consumers to only run the tests, and skip the unrelated logic in test_*_{cpp,python}.sh.