[checkbox-ce-oem] Add a test for splash screen (New)

[vandah]

Description

This PR adds a manual test for checking if a splash screen appears on boot. To do this, the device is first rebooted and the result is recorded upon resuming the Checkbox session. The rebooting method is the command from power-management/warm-reboot.

Tests

• tested on a Raspberry Pi 4 Model B with UC22

[vandah]

Please review, @alexclewontin.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.84%. Comparing base (df70356) to head (6ab8185).
Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1790   +/-   ##
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  Coverage   49.84%   49.84%           
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  Files         377      377           
  Lines       40723    40723           
  Branches     6849     6849           
=======================================
  Hits        20299    20299           
  Misses      19700    19700           
  Partials      724      724           

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