Refactor test cases to improve unit test quality

[freddiewanah]

Hello, first-time submitting PR here.

This PR improves the readability of the assert statements in some of the unit tests by using more expressive assert methods from the unittest module. These changes improve code quality (avoid test smells) by:

• Making the assert statements more readable, e.g., the use of assertTrue, assertFalse, assertRaises.

• Simplifying code management. For example, instead of simply saying "this", use the parameter name(UserDefinedFilesPipeline.DEFAULT_FILES_RESULT_FIELD) so that developers could only update the value in one place.

Happy to update more related issues in the test code if needed :)

[codecov]

Codecov Report

Merging #5986 (858ce04) into master (af2aa4b) will not change coverage.
The diff coverage is n/a.

❗ Current head 858ce04 differs from pull request most recent head 389da53. Consider uploading reports for the commit 389da53 to get more accurate results

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  Coverage   89.21%   89.21%           
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  Files         162      162           
  Lines       11301    11301           
  Branches     1835     1835           
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  Hits        10082    10082           
  Misses        927      927           
  Partials      292      292           

[freddiewanah]

Hi wRAR, thanks for the comments. I've reverted the updates about all the assertRaises.

[freddiewanah]

Update the assert format with black.

[Gallaecio]

Nice cleanup, thanks!