stat test is flakey

Nobody has claimed this yet.

### What is the issue?

Timed out in `external_prometheus_deep` suite with the following error:

```
Error: TestCliStatForLinkerdNamespace - timed-out checking stats (20s)
--- FAIL: TestCliStatForLinkerdNamespace (36.05s)
--- FAIL: TestCliStatForLinkerdNamespace/linkerd_viz_stat_svc_-n_linkerd-stat-test (20.33s)
stat_test.go:269: Error stripping header and trailing newline; full output:
```

https://github.com/linkerd/linkerd2/runs/4887651353?check_suite_focus=true

**Edit**: seen again

https://github.com/linkerd/linkerd2/runs/4954477559?check_suite_focus=true
### How can it be reproduced?

Bad luck :( N/A

### Logs, error output, etc

N/A

### output of `linkerd check -o short`

N/A

### Environment

N/A

### Possible solution

_No response_

### Additional context

_No response_

### Would you like to work on fixing this bug?

_No response_

Start with the external_prometheus_deep suite and the failure at stat_test.go:269, then inspect the linked GitHub Actions runs for the full output and timing. Determine why TestCliStatForLinkerdNamespace can exceed the 20-second check and verify that the stat test passes reliably without the timeout.

Written by the indexing model from the issue text.