Improves tests from #12362
These tests try to setup the following concurrent scenario:
1. (goroutine 1) execute read RPC with index=0
2. (goroutine 1) get response from (1) @ index=10
3. (goroutine 1) execute read RPC with index=10 and block
4. (goroutine 2) WHILE (3) is blocking, start slamming the system with stray writes that will cause the WatchSet to wakeup
5. (goroutine 2) after doing all writes, shut down the reader above
6. (goroutine 1) stops reading, double checks that it only ever woke up once (from 1)
