Due to PR(sonic-net/sonic-buildimage#12572) changes utils.py:run_command behavior, so that the system_health/test_system_health.py:test_external_checker always test failure. I try to add code to show the error message as below.

root@sonic:/home/admin# /usr/local/bin/healthd
[Errno 2] No such file or directory: 'cat /tmp/mock_valid_external_checker.txt'

Steps to reproduce the issue:
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Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```