Improve tests for classes related to typing of tabular data

[lars-reimann]

Tests for methods of

• DataType (interface) and _PolarsDataType (implementation),
• Schema (interface) and _PolarsSchema (implementation)

are missing entirely or for some edge cases.

We should

• remove the outdated test_data_type.py and test_schema.py
• create folders _data_type and _schema inside tests/safeds/data/tabular/typing
• create individual files inside the folder for the different methods of DataType and Schema (same structure as for containers of tabular data).

Use parametrized tests to cover edge cases.

[lars-reimann]

🎉 This issue has been resolved in version 0.30.0 🎉

The release is available on:

• v0.30.0
• GitHub release

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