[checkbox-ce-oem] Enhance serial test to be more stable (Bugfix)

[hanhsuan]

Description

1. The server mode shouldn't be exited while getting the payload that couldn't be decoded.
2. The client mode should clean the buffer before starting the test to avoid receiving non testing payload.

Resolved issues

Documentation

Tests

https://certification.canonical.com/hardware/202405-34046/submission/408239/

[rickwu666666]

LGTM, Thanks for the patch.

[LiaoU3]

Thank you for the patch!

[hanhsuan]

@rickwu666666 mention the client mode couldn't work on his side, let's check it before merging.

[rickwu666666]

I've tried the change on two different projects that have RS485. And works fine.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.99%. Comparing base (b1dfd6f) to head (d3842b7).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1682   +/-   ##
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  Coverage   48.99%   48.99%           
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  Files         372      372           
  Lines       40321    40321           
  Branches     6811     6811           
=======================================
  Hits        19757    19757           
  Misses      19842    19842           
  Partials      722      722           

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