[7.0] Write functional tests to validate consumable results

[Jonathing]

ForgeGradle 7.0 and later need to have functional tests written for it to avoid regressions that may be caused by updates to the Minecraft Mavenizer, Slime Launcher or ForgeGradle itself.

This is especially important in terms of how ForgeGradle handles Mixin and its compatibility with projects. Earlier incidents (#1026 is not the only one) have proven that there are unbearably high expectations on ForgeGradle to always remain functional with regards to Mixin and its usage in projects and at runtime. Any potential regressions to Mixin functionality in ForgeGradle threaten to be a massive headache moving forward. This needs to be addressed and tests need to be made.

Note

Gradle's User Guide has a page on Testing Plugins. This will serve as a good starting point to write tests for all of our projects, including ForgeGradle, though all of our projects should get the same love with regards to tests and buildscript cleanliness.

[LexManos]

As an extension, The MDKExamples repo is intended to eventually be full blow tests. Showing both how to do things in Gradle, but also Game Tests as well.
Eventually, I would like to have a standardized layout for this repo, and all the projects setup in a multi-project build so that we can run automated tests. greadlew fg7:test runs all tests for FG7 for example.

But for now, the basic layout is {Subject}/fg{Version} as standalone projects.