memtable_snapshot_source starts a background fiber in its constructor,
which compacts LSA memory in a loop. The loop's inside is covered with a
critical alloc section. It also contains a wait on a condition variable
and in its present form the critical section also covers the wait,
effectively turning off allocation failure injection for any test using
the memtable_snapshot_source.
This patch disables the critical alloc section while the loop waits on
the condition variable.
ffefa623f4