* Fix ESP32 platform manager shutdown.

The QEMU ESP32 tests start and stop the platform manager several times, and the
lack of a shutdown implementation was putting things in a bad state.

Also adjusts the test runner script to trigger a test failure when we do end up
in that bad state: we were passing even though we crashed quite early in the
test run and most of the tests did not run.

* Address review comments.

* Fix unlocking in FreeRTOS _RunEventLoop.

Ensure we always start with an empty event queue after FreeRTOS _InitChipStack.