Add MockOnlyRule to allow skipping specific tests based on downloader

[XiangRongLin]

• I carefully read the contribution guidelines and agree to them.

-

• addresses point 7 of Automated testing strategy in NewPipeExtractor #481
• from @Rule allow to ignore Tests junit-team/junit4#116

showcase

• changes: https://github.com/XiangRongLin/NewPipeExtractor/commits/flaky_test_showcase
• Run that branch locally with gradle check -Ddownloader=REAL and gradle check -Ddownloader=MOCK`
  • Mock will fail because failing test is not skipped
  • REAL will succeed because failing test is skipped

[XiangRongLin]

After thinking about it a bit more i think adding a custom test runner is better.

• There is an additional overhead of specifiying the customer test runner
• But it can be easily added on method level, instead of class level

[XiangRongLin]

With rules it's also possible to skip tests on method level instead of class level. Other that that it's the same addtional overhead compared to the developer to add it. But it allows to use other test runner (Mockito).

I adjusted the showcase branch

[Stypox]

@XiangRongLin can this be merged? If yes, feel free to do it yourself 😉