Restore TeachableAgent tests

[rickyloynd-microsoft]

Why are these changes needed?

• The pytest testing of TeachableAgent was disabled in the recent API update. This PR restores the tests, now located in the test/contrib folder.
• A new test is added to verify that the accuracy of teachability does not drop significantly.

Related issue number

Closes #450

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 ✅

Comparison is base (d8d0527) 27.81% compared to head (67d792e) 48.85%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #761       +/-   ##
===========================================
+ Coverage   27.81%   48.85%   +21.03%     
===========================================
  Files          27       27               
  Lines        3480     3480               
  Branches      789      831       +42     
===========================================
+ Hits          968     1700      +732     
+ Misses       2441     1603      -838     
- Partials       71      177      +106     

Flag	Coverage Δ
unittests	48.59% <ø> (+20.83%)	⬆️

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