test: improve structure of tests for Column

[lars-reimann]

Closes #148.

Summary of Changes

• One test file per method (putting them all in one file would be unwieldy)
• Add missing tests

[codecov]

Codecov Report

Merging #246 (6e79519) into main (a35b943) will increase coverage by 0.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
+ Coverage   98.62%   99.20%   +0.58%     
==========================================
  Files          44       44              
  Lines        1523     1518       -5     
==========================================
+ Hits         1502     1506       +4     
+ Misses         21       12       -9     

Impacted Files	Coverage Δ
src/safeds/data/tabular/containers/_column.py	100.00% <100.00%> (+5.23%)	⬆️

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[lars-reimann]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	38	0	0	1.19s
✅ PYTHON	mypy	38		0	2.75s
✅ PYTHON	ruff	38	0	0	0.07s
✅ REPOSITORY	git_diff	yes		no	0.03s

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[lars-reimann]

🎉 This PR is included in version 0.12.0 🎉

The release is available on:

• v0.12.0
• GitHub release

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