This turned out to be a problem in the tests. The tests used to do
1. allocate
2. increment
3. free
4. decrement
But if one goroutine had just completed 2 and another had just
completed 3 then this can cause the test to register too many
allocations.
This was fixed by doing the test in this order instead:
1. allocate
2. increment
3. decrement
4. free
The 4 operations are atomic.
Fixes#8813
b5cbb7520d