Add test to check examples are found by cleanlab

[weijinglok]

Add test to check that highlighted examples are found by cleanlab's find_label_issues in the tutorials.

1. A mockup is available here - visually should have no differences from how it already is.
2. An example of a failed build (i.e., checking an index that doesn't exist) is available here.
3. Examples of successful builds are available here and here.

Commits were not squashed for traceability and audit. Please squash them before merging.

[codecov]

Codecov Report

Merging #172 (c7ee174) into master (dcdb890) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   87.64%   87.65%   +0.01%     
==========================================
  Files          12       12              
  Lines        1028     1021       -7     
  Branches      194      192       -2     
==========================================
- Hits          901      895       -6     
  Misses        104      104              
+ Partials       23       22       -1     

Impacted Files	Coverage Δ
cleanlab/internal/util.py	100.00% <0.00%> (ø)
cleanlab/example_models/mnist_pytorch.py	99.27% <0.00%> (+0.79%)	⬆️

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[jwmueller]

LGTM!