chore(tests): add tests for converter methods

[taiyeoguns]

Fixes Issue

Resolves #277

Description

Adds tests for convert methods

Check List (Check all the applicable boxes)

• I understand that my contributions needs to pass the checks.
• If I created new functions / methods, I documented them and add type hints.
• If I modified already existing code, I updated the documentation accordingly.
• The title of my pull request is a short description of the requested changes.

Screenshots

Note to reviewers

[jeertmans]

Thanks for your PR @taiyeoguns! I'll review and merge this once I have some time :-)

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
manim_slides/__main__.py	71.42% <100.00%> (-0.67%)	⬇️
manim_slides/config.py	83.91% <ø> (ø)
manim_slides/docs/manim_slides_directive.py	0.00% <ø> (ø)
manim_slides/logger.py	100.00% <100.00%> (ø)
manim_slides/slide/manimlib.py	100.00% <100.00%> (ø)
manim_slides/wizard.py	37.39% <ø> (ø)
manim_slides/convert.py	87.50% <66.66%> (+13.31%)	⬆️
manim_slides/slide/animation.py	96.55% <96.55%> (ø)
manim_slides/utils.py	89.74% <83.33%> (-2.15%)	⬇️
manim_slides/slide/manim.py	93.47% <93.47%> (ø)
... and 2 more

📢 Thoughts on this report? Let us know!.

[jeertmans]

Looks good to me, thanks @taiyeoguns!