fix: hermes test hardcoding python3 (fails on Windows) (#513)

tests/hermes-plugin.test.js spawned python3 with no fallback. On Windows
python3 is the Store app-execution-alias stub that exits non-zero even when
Python is installed. Reuse the probe/fallback pattern from
benchmarks/correctness.js: try python3, fall back to python, cache the result.

Fixes #498