Migrate unit tests away from Spock to JUnit back

[php-coder]

Rationale:

• less languages/frameworks to learn => easier start for contributors
• less languages/frameworks to support/maintain => less headache to me (CodeNarc and its plugin have issues: #508, #539, #994), less time I need to spend on this (for example, updating Gmaven plugin (#958), Groovy (#938), Spock (#761), CodeNarc (#1187), integration to our CI (#1479))
• I noticed that tests on Spock/Groovy are running slower in IDEA than JUnit ones
• The community seems inactive so the future is unclear and features/updates get released longer
• JUnit has rich ecosystem
• I found that syntax sugar doesn't outweight all aforementioned

TODO:

• [ ] SlugUtilsTest
• [ ] LocaleUtilsTest
• [ ] PagerTest
• [ ] SeriesSalesServiceImplTest
• [ ] CatalogUtilsTest
• [ ] StampsCatalogServiceImplTest
• [ ] SeriesSalesImportServiceImplTest
• [ ] SeriesInfoExtractorServiceImplTest
• [ ] TimedSeriesInfoExtractorServiceTest
• [ ] SeriesImportServiceImplTest
• [ ] SeriesServiceImplTest
• [ ] CollectionServiceImplTest
• [ ] CategoryServiceImplTest
• [ ] ParticipantServiceImplTest
• [ ] FilesystemImagePersistenceStrategyTest
• [ ] ImageServiceImplTest
• [ ] TimedImagePreviewStrategyTest
• [ ] DatabaseImagePersistenceStrategyTest
• [ ] SiteServiceImplTest
• [ ] CronServiceImplTest
• [ ] SuspiciousActivityServiceImplTest
• [ ] UsersActivationServiceImplTest
• [ ] UserServiceImplTest
• [ ] CountryServiceImplTest
• [ ] Remove plugin, dependencies, build logic, Danger support, update editorconfig
• [ ] Close Groovy/Spock-related issues
• [ ] Update README/NEWS
• [ ] remove dependency on junit-jupiter-engine and exclude (and ban?) junit from other dependencies
• [ ] remove a comment for #761

[php-coder]

Heh, Spring Security has been also migrated away of Groovy: https://github.com/spring-projects/spring-security/issues/4939

[php-coder]

Good comparison: https://blog.solidsoft.pl/2020/04/15/spock-vs-junit-5-the-ultimate-feature-comparison/

[php-coder]

Perhaps we can use @TestTemplate -- https://www.baeldung.com/junit5-test-templates

[php-coder]

https://www.arhohuttunen.com/junit-5-mockito/