test: improve ml tests

[lars-reimann]

Closes #116.
Closes #117.

Summary of Changes

• Reduce code duplication
• Test that fit doesn't change the input table
• Test that predict doesn't change the input table

[lars-reimann]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	2	0	0	0.61s
✅ PYTHON	flake8	2		0	0.36s
✅ PYTHON	isort	2	0	0	0.22s
✅ PYTHON	mypy	2		0	1.53s
✅ PYTHON	pylint	2		0	1.35s
✅ REPOSITORY	git_diff	yes		no	0.03s

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[codecov]

Codecov Report

Merging #129 (a624465) into main (ddd3f59) will increase coverage by 0.07%.
The diff coverage is n/a.

❗ Current head a624465 differs from pull request most recent head eee1d9e. Consider uploading reports for the commit eee1d9e to get more accurate results

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
+ Coverage   93.14%   93.22%   +0.07%     
==========================================
  Files          36       36              
  Lines        1254     1254              
==========================================
+ Hits         1168     1169       +1     
+ Misses         86       85       -1     

see 1 file with indirect coverage changes

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[lars-reimann]

🎉 This PR is included in version 0.8.0 🎉

The release is available on:

• v0.8.0
• GitHub release

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