Test for #7815

This test differes slightly from the code in the ticket, but the
simplification betters the chances of seeing the error.  Instead of
looking for the error with an additional tranaction, when error appears
only one `reset` can complete and the other is blocked on STM raising an
exception.
Whitespace only
Add a multi_cpu_race helper

This makes the test run alone, to increase the chance of a
multi-CPU race happening.