Fix Concurrency issues in test 1941

The tracing support code we were using in test 1941 was not
thread-safe. This led to the test sometimes failing in odd ways. This
fixes the support code and also updates the 1941 test itself to make
it somewhat more focused.

Bug: 70459579
Test: ./test.py -j50 --host
Change-Id: If9e5250bc233a407ee678b89d4dffd52baebdf5d