Enables JUnit Jupiter best practices

[koppor]

This enables JUnit Jupiter best practices

Two main changes:

• Statically import JUnit Jupiter assertions
• Remove public visibility of JUnit 5 tests, which is SonarSource Rule S5786

(Follow-up to #11642)

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Fetcher tests for quey transformation start to fail - need to investigate

---

org.jabref.logic.importer.fetcher.transformers.ArXivQueryTransformerTest
  Test convertYearRangeField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[]> but was: <Optional.empty>

  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[2018]> but was: <Optional[all:2018]>

org.jabref.logic.importer.fetcher.transformers.CollectionOfComputerScienceBibliographiesQueryTransformerTest
  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[year:2018]> but was: <Optional[2018]>

org.jabref.logic.importer.fetcher.transformers.DBLPQueryTransformerTest
  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[2015]> but was: <Optional.empty>

  Test convertYearRangeField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[2012|2013|2014|2015]> but was: <Optional.empty>

org.jabref.logic.importer.fetcher.transformers.DefaultQueryTransformerTest
  Test convertYearRangeField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[]> but was: <Optional.empty>

  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[]> but was: <Optional.empty>

org.jabref.logic.importer.fetcher.transformers.GVKQueryTransformerTest
  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[ver:2018]> but was: <Optional[pica.erj=2018]>

org.jabref.logic.importer.fetcher.transformers.JstorQueryTransformerTest
  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[sd:2018 AND ed:2018]> but was: <Optional[sd:2018 AND ed: 2018]>

org.jabref.logic.importer.fetcher.transformers.ScholarQueryTransformerTest
  Test convertYearRangeField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[]> but was: <Optional.empty>

  Test convertYearField() FAILED
  org.opentest4j.AssertionFailedError: expected: <Optional[]> but was: <Optional.empty>

[koppor]

OK, we forgot to add @Test annotations. The tests were never executed.

[koppor]

All should be good now. #11644 is kind of a pre-condition to get the OK checkmark for FetcherTests here, too.

[koppor]

All should be good now. #11644 is kind of a pre-condition to get the OK checkmark for FetcherTests here, too.

We merged - could not fix one test. This test is also red here, all others green. Thus, this is good to go.

[github-actions]

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