Use python interpreter that executed atest/run.py instead of python

[emanlove]

When running under a virtualenv, using python as the interpreter forgets the virtualenv python which executed atest.run.py. Instead using sys.executable which will execute with same interpreter used to execute atest/run.py. I also later check to make sure the interpreter is not None nor an empty string as doc states [1] it may be if "Python is unable to retrieve the interpreter".

Fixes #1796

[1] https://docs.python.org/3.10/library/sys.html#sys.executable