Adds back the JUnit testing system

[TheLimeGlass]

Description

Adds back the JUnit testing system.

• Adds JUnit tests under src/test/java again.
• All tests under src/test/java are ran with the test runner server using JUnit.
• Renamed ch.njol.skript.tests to ch.njol.skript.test so that static loadClasses can read ch.njol.skript.test.tests without other classes interfering.
• The old JUnit testing used to run each time the jar was built, this was annoying but not an issue. This pull request keeps the runtime tests separate from the gradlew clean build task.
• Cleaned up the formatting of the test results to be much better. (Failed tests could have been displayed as success)
• Gradle tasks are JUnitJava17, JUnitJava8 and JUnitQuick (Capitalization doesn't matter)
• Added a folder in the test scripts named "junit" and a readme explaining how to do JUnit scripts combined with Java classes.
• Reduced the test failure timeout to 5 minutes.
• All exceptions that happen in the JUnit tests prints as Skript.exception formatted.
• Added expression [the] [current[ly [running]]] junit test [name] for test scripts to be able to figure out what JUnit test is being ran currently. This opens the possibility of anything to be done with Java to Skript for testing.
• Moves the /test_runners folder into /build/test_runners so that you can use gradlew clean or no clean if you don't want to clear the build cache. (Pickle and I wanted this in a conversation)
• Adds an objective completeness system. A script defines string objectives that the JUnit needs to complete in conjunction with any runtime things the test is doing. New syntax at EffObjectives in test runner package.

Notes:

• This is superior to Implementation of Java tests #4670 because it actually uses the JUnit library and runs along side the test runner server without needing a script .sk existing, just a typical JUnit class.
• JUnit 4 is the only version that has the JUnitCore runner that allows runtime unit checking, so we have to use JUnit 4, which is fine, we just need standard JUnit functionality.
• Adds EasyMock support Usage of EasyMock in Skript tests (Create fake objects like Players) #5033
• The GitHub actions are split into two checks for speed completion.

---

Related Issues: #4973

[kiip1]

All good! When #5299 is merged you can remove some duplicate code as TP said.

[Moderocky]

Looks good :)

[AyhamAl-Ali]

Well done 🚀

[AyhamAl-Ali]

Looking good 👏

[APickledWalrus]

Looks good. Once TP's comments are addressed and his approval is obtained, this should be good to go.

Applied all changes