Fix 2393-virtual-thread-pinning-monitor

The virtual thread in the test could be unparked by the test thread scheduler on another carrier before completely unmounted from the first carrier. It's trying to fix it by waiting for the first carrier thread to join.

Also, improve the debug logs if a virtual thread is mounted twice,
or is absent when being removed from the linked list of
mounted virtual threads virtual_and_carrier_map_.

Flag: EXEMPT BUGFIX
Bug: 492154287
Test: testrunner.py
Change-Id: I0324e7ab1408d2b64038935cda506a6dc954b4d8