fallback on python versions when running e2e-test
fallback on python versions when running e2e-test

Summary: Instead of relying on the shebang in `tests.py`, try to figure out the correct python interpreter to use before launching the test.

Reviewed By: perryjrandall

Differential Revision: D4705406

fbshipit-source-id: c18865ff259fa05f64c3a446675100dafdc75c2d

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