Use proper python executable and paths

[maludwig]

When running tests with pytest:

pytest tests/inference

The test runner was just using "python" which works for when the system python is the thing running the tests. It doesn't work if the tests are run inside a virtualenv. Or if a different python was used.

This now uses the same python executable that the test files are running on, respecting virtualenvs, or alternate python installs.

It also fixes the paths in the tests/inference/extra_model_paths.yaml file to be properly mapped. The path should be relative to the extras file.