It's followup of #15667.
This patch is to use zaptest/observer as base to provide a similar
function to pkg/expect.Expect.
The test env
```bash
11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
mkdir /sys/fs/cgroup/etcd-followup-15667
echo 0-2 | tee /sys/fs/cgroup/etcd-followup-15667/cpuset.cpus # three cores
```
Before change:
* memory.peak: ~ 681 MiB
* Elapsed (wall clock) time (h:mm:ss or m:ss): 6:14.04
After change:
* memory.peak: ~ 671 MiB
* Elapsed (wall clock) time (h:mm:ss or m:ss): 6:13.07
Based on the test result, I think it's safe to be enabled by default.
Signed-off-by: Wei Fu <fuweid89@gmail.com>
