Add environment-agnostic scripts for running ctests and pytests

[trxcllnt]

Description

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.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [ ] The documentation is up to date with these changes.