Add a unit test for the tag_proto_objects method

[achals]

Signed-off-by: Achal Shah [email protected]

What this PR does / why we need it:
This PR just adds tests for a fairly critical piece of code.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

none

[codecov-commenter]

Codecov Report

Merging #2163 (062eb4a) into master (a710cb9) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2163      +/-   ##
==========================================
+ Coverage   84.39%   84.45%   +0.05%     
==========================================
  Files         101      102       +1     
  Lines        8133     8163      +30     
==========================================
+ Hits         6864     6894      +30     
  Misses       1269     1269              

Flag	Coverage Δ
integrationtests	74.18% <16.66%> (-0.51%)	⬇️
unittests	58.88% <100.00%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/tests/unit/diff/test_fco_diff.py	100.00% <100.00%> (ø)

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[APPROVALNOTIFIER] This PR is APPROVED

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