Refactor Datumaro format code and test code

[vinnamkim]

Summary

• Related ticket: 104694
• This PR is required as a pre-work for the Datumaro binary format implementation task (104650). It will refactor Datumaro format to make a room for Datumaro binary format implementation.
• This also migrate our testing framework from unittest to pytest in Datumaro unit test code. Please see the reason why we move on as follows.

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Currently, Datumaro uses old-fashioned unittest framework and its unit test codes has been time-worn so much. There have been super many duplicated and never re-use codes in our tests. In addition, if we keep using unittest, it is expected that the spaghetti with no taste will continue to grow its size bigger and bigger. This epic aims to migrate our testing framework from unittest to pytest. Appropriate code refactoring and use of pytest fixtures can slim down our testing code.

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How to test

This PR's test refactoring will cover.

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Base: 78.27% // Head: 78.33% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (e936086) compared to base (a3bd7de).
Patch coverage: 96.87% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #824      +/-   ##
===========================================
+ Coverage    78.27%   78.33%   +0.06%     
===========================================
  Files          184      189       +5     
  Lines        23614    23681      +67     
  Branches      4892     4895       +3     
===========================================
+ Hits         18484    18551      +67     
  Misses        4032     4032              
  Partials      1098     1098              

Flag	Coverage Δ
macos-11_Python-3.8	77.64% <96.87%> (+0.06%)	⬆️
ubuntu-20.04_Python-3.8	78.31% <96.87%> (+0.05%)	⬆️
windows-2019_Python-3.8	78.25% <96.87%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
...o/plugins/data_formats/datumaro_binary/importer.py	86.66% <86.66%> (ø)
datumaro/plugins/data_formats/datumaro/exporter.py	93.02% <93.33%> (+0.06%)	⬆️
datumaro/components/importer.py	89.68% <100.00%> (+0.25%)	⬆️
datumaro/plugins/data_formats/datumaro/base.py	92.96% <100.00%> (-0.65%)	⬇️
datumaro/plugins/data_formats/datumaro/format.py	100.00% <100.00%> (ø)
datumaro/plugins/data_formats/datumaro/importer.py	100.00% <100.00%> (ø)
...umaro/plugins/data_formats/datumaro_binary/base.py	100.00% <100.00%> (ø)
...o/plugins/data_formats/datumaro_binary/exporter.py	100.00% <100.00%> (ø)
...aro/plugins/data_formats/datumaro_binary/format.py	100.00% <100.00%> (ø)
datumaro/util/test_utils.py	85.29% <100.00%> (+1.16%)	⬆️
... and 2 more

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