update test for resume_from_checkpoint on missing file

[awaelchli]

What does this PR do?

Follow up to #7075

Updates the test docstring
Updates the test to test the file not found error only
Adds missing changelog

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• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

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[codecov]

Codecov Report

Merging #7255 (a12c1c8) into master (f135deb) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #7255   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         200     200           
  Lines       12895   12895           
======================================
  Hits        11259   11259           
  Misses       1636    1636           

[ananthsub]

👍

[carmocca]

@awaelchli GPU failure looks real

[tchaton]

LGTM !

[awaelchli]

@awaelchli GPU failure looks real

yeah looks real from master xD