Change how we put test libraries on the class path

- Mark our core test-fixtures jar as part of the "cctest", rather than
   a separate library. I'm fairly sure this was actually using the
   classpath version of CC rather than the legacyClasspath version!

 - Add a new "testMinecraftLibrary" configuration, instead of trying to
   infer it from the classpath. We have to jump through some hoops to
   avoid having multiple versions of a library on the classpath at once,
   but it's not too bad.

I'm working on a patch to bsl which might allow us to kill of
legacyClasspath instead. Please, anything is better than this.

projects/core/build.gradle.kts

@@ -50,13 +50,6 @@ tasks.test {
     systemProperty("cct.test-files", layout.buildDirectory.dir("tmp/testFiles").getAbsolutePath())
 }
 
-tasks.testFixturesJar {
-    manifest {
-        // Ensure the test fixtures jar loads as a mod. Thanks FML >_>.
-        attributes("FMLModType" to "GAMELIBRARY")
-    }
-}
-
 val checkChangelog by tasks.registering(cc.tweaked.gradle.CheckChangelog::class) {
     version.set(modVersion)
     whatsNew.set(file("src/main/resources/data/computercraft/lua/rom/help/whatsnew.md"))