Fix flaky test tests/test_microvmd_vz.py::test_stop_vz_kills_a_helper_that_ignores_both_sigterms

The merge lane saw `tests/test_microvmd_vz.py::test_stop_vz_kills_a_helper_that_ignores_both_sigterms` fail and then pass on a rerun of the same tree. - Test: `tests/test_microvmd_vz.py::test_stop_vz_kills_a_helper_that_ignores_both_sigterms` - Tree: `a21aec53e8af009baa97e5de4dfbf682062f8d63` - Lane task: 11735 - Flake record: 01a18fa1-ca91-736a-8745-9e9d7be160ea Make it deterministic; do not delete it unless it tests nothing.

Root cause: _stop_vz sent a 3rd SIGTERM before SIGKILL (via _terminate). Now calls new _kill directly; test spies on sent signals. Under load the old code failed 7/120 and the new one 0/120. Quick gate green; PR #7467 queued in the lane. Memory: none applied.