Adds an entrypoint for use by JUnit infrastructure

[shartte]

Depends on McModLauncher/securejarhandler#60

The goal is to add a new entrypoint to BSL for JUnit runners, which disables classloader isolation. This change adds that entrypoint. It is intentionally called testMain to prevent it from being used accidentally via the command-line.

This is used to allow test-classes to live in the GAME layer, while still loading JUnit and other testing library classes found on the normal classpath. When being run by a unit testing framework, we do not control the JVM entrypoint like we normally do, and Gradle for example will class-load JUnit before calling any other hook. Our test classes need to refer to those same loaded classes, making it easier if isolation is disabled in such scenarios.

[marchermans]

I have thought about this for a while.

Allthough I am convinced that the JUnit API provides other, more standardized, ways of doing testing in a modularly isolated environment, right now we do not have the time.

As such this is at least a bridge and a gap to allow for this to work.

However, since I want to be clear that this should not be used in production, what so ever, I want a very load warning in the source code, in the form of previously mentioned annotations and documentation. As well as, a very loud warning in a log file!

Please add those and then this is good.