IIUC, using runpy (either from Python or in a subprocess i.e. python -m test.regrtest) would allow running from build dir.

Coverage reports are nice, but I personally dislike the idea of code downloading something via HTTP and running it. I’d prefer just having an optional dependency on it and letting the user install it if they want.

Code review:
1) There is already a popular (I think) module named coverage, please use another name.
1.1) How does figleaf compare with coverage?
2) Please use tarfile instead of running tar (not always available).
3) Please use the Python API instead of running subprocesses.
4) subprocess is recommended over os.system. Neither will be required if you agree to 2 and 3 :)
5) Please make the code PEP 8-compliant.

Hope this helps! :)