[Datumaro] Reducing nesting of tests

[KochankovID]

Motivation and context

Tests are complex. It's necessary to reduce nesting.
To achieve this goal was implemented Dataset.from_iterable(), labelCategories.from_iterable()
and PointsCategories.from_iterable(). The format tests was simplified.

How has this been tested?

It was tested using unittests.

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Pull Request Test Coverage Report for Build 6397

• 27 of 29 (93.1%) changed or added relevant lines in 2 files are covered.
• 42 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.2%) to 65.03%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datumaro/datumaro/components/extractor.py	16	18	88.89%

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	1	77.35%
datumaro/datumaro/components/extractor.py	1	90.96%
datumaro/datumaro/cli/main.py	11	0%
datumaro/datumaro/plugins/voc_format/converter.py	29	88.67%

Totals
Change from base Build 6342:	-0.2%
Covered Lines:	11096
Relevant Lines:	16656

---

💛 - Coveralls

[zhiltsov-max]

Could you fill the PR description?

[zhiltsov-max]

LGTM, please fix conflicts so it could be merged.

[nmanovic]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- datumaro/tests/test_mot_format.py  1
- datumaro/tests/test_coco_format.py  3
         

See the complete overview on Codacy

[zhiltsov-max]

Looks nice.