Update vision capability test: create dummy image before testing

[BeibinLi]

Why are these changes needed?

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.57%. Comparing base (c27eb0d) to head (d235a0d).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2258       +/-   ##
===========================================
+ Coverage   37.82%   49.57%   +11.75%     
===========================================
  Files          77       77               
  Lines        7784     7784               
  Branches     1667     1805      +138     
===========================================
+ Hits         2944     3859      +915     
+ Misses       4589     3599      -990     
- Partials      251      326       +75     

Flag	Coverage Δ
unittest	14.36% <ø> (?)
unittests	48.50% <ø> (+10.68%)	⬆️

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