Fix links in Explanation-Formats

[vinnamkim]

Summary

• Find and replace: https://github.com/openvinotoolkit/datumaro/blob/develop/tests/test_ -> https://github.com/openvinotoolkit/datumaro/blob/develop/tests/unit/test_
• Fix common_semantic_segmentation.md's test link

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (13fd698) 78.68% compared to head (2f248d7) 78.69%.

Additional details and impacted files

@@               Coverage Diff               @@
##           releases/1.2.0     #965   +/-   ##
===============================================
  Coverage           78.68%   78.69%           
===============================================
  Files                 231      231           
  Lines               26420    26420           
  Branches             5258     5258           
===============================================
+ Hits                20789    20790    +1     
+ Misses               4403     4402    -1     
  Partials             1228     1228           

Flag	Coverage Δ
ubuntu-20.04_Python-3.8	78.69% <ø> (+<0.01%)	⬆️

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see 6 files with indirect coverage changes

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