ADD: Add tests to improve coverage

[ldevillez]

Description

The tests need to cover the whole code to be really useful. It is also a step toward releasing svg2tikz directly with inkscape.
Fixes #187 and also #99 (tests could be reworked but it will be okay for now).

The test github action is modify to show the coverage.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Coverage was tested with:

python3 -m coverage run -m unittest

And analyzed with:

python3 -m coverage html

New tested were tested visually.

Checklist:

• My code follows the style guidelines of this project (black/pylint)
• I have updated the changelog with the corresponding changes
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[ldevillez]

Do not merge before #194 for an easier integration

[github-actions]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
1107	1100	99%	0%	🟢

New Files

No new covered files...

Modified Files

File	Coverage	Status
svg2tikz/init.py	100%	🟢
svg2tikz/tikz_export.py	99%	🟢
tests/test_complete_files.py	99%	🟢
tests/test_geometrical_functions.py	100%	🟢
tests/test_parsing.py	100%	🟢
tests/test_svg2tikz.py	100%	🟢
tests/test_tikz_path_exporter.py	100%	🟢
tests/test_utility_functions.py	100%	🟢
TOTAL	100%	🟢

updated for commit: e0c9fff by action🐍