build: Fix JUnit trying to test Mixin classes

Also added `abstract` modifiers to all mixin test classes, and did a small local variable rename.

build.gradle

@@ -263,6 +263,7 @@ test {
         events "passed", "skipped", "failed"
     }
     jvmArgs("-ea", "-Dmixin.debug.verbose=true", "-Dmixin.debug.export=true", "-Dmixin.checks.interfaces=true")
+    exclude "**/mixin*"
 }
 
 tasks.withType(ProcessResources).configureEach {

src/test/java/io/github/opencubicchunks/cubicchunks/mixin/test/common/MixinDefaultAttributesTest.java

@@ -8,7 +8,7 @@
 import org.spongepowered.asm.mixin.injection.Redirect;
 
 @Mixin(DefaultAttributes.class)
-public class MixinDefaultAttributesTest {
+public abstract class MixinDefaultAttributesTest {
     @Redirect(method = "<clinit>", at = @At(value = "FIELD", target = "Lnet/minecraft/world/entity/ai/attributes/DefaultAttributes;SUPPLIERS:Ljava/util/Map;", shift = At.Shift.BY, by = -1))
     private static ImmutableMap<?, ?> cubic_chunks_3$fixNeoForgeRegistryError(ImmutableMap.Builder<?, ?> instance) {
         return ImmutableMap.builder().build();

src/test/java/io/github/opencubicchunks/cubicchunks/mixin/test/common/MixinPlayerTest.java

@@ -8,7 +8,7 @@
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
 
 @Mixin(Player.class)
-public class MixinPlayerTest {
+public abstract class MixinPlayerTest {
     @Inject(method = "createAttributes", at = @At(value = "HEAD"), cancellable = true)
     private static void cubic_chunks_3$fixNeoForgeErrors(CallbackInfoReturnable<AttributeSupplier.Builder> cir) {
         cir.setReturnValue(AttributeSupplier.builder());

src/test/java/io/github/opencubicchunks/cubicchunks/mixin/test/common/server/MixinServerPlayerTest.java

@@ -7,7 +7,7 @@
 import org.spongepowered.asm.mixin.Shadow;
 
 @Mixin(ServerPlayer.class)
-public class MixinServerPlayerTest {
+public abstract class MixinServerPlayerTest {
     @Shadow
     @SuppressWarnings("unused")
     public ServerGamePacketListenerImpl connection = Mockito.mock();

src/test/java/io/github/opencubicchunks/cubicchunks/test/server/level/TestServerPlayer.java

@@ -36,11 +36,11 @@ private ServerPlayer setupServerPlayer(ServerLevel serverLevel) {
             ServerPlayer player = setupServerPlayer(serverLevelReference.value());
 
             // teleportTo takes the destination level to teleport to as a param, so we mock it
-            ServerLevel serverLevelReferenceSpy = spy(serverLevelReference.value());
+            ServerLevel serverLevelSpy = spy(serverLevelReference.value());
             ServerChunkCache serverChunkCacheMock = mock(ServerChunkCache.class);
-            Mockito.when(serverLevelReferenceSpy.getChunkSource()).thenReturn(serverChunkCacheMock);
+            Mockito.when(serverLevelSpy.getChunkSource()).thenReturn(serverChunkCacheMock);
 
-            player.teleportTo(serverLevelReferenceSpy, 0, 0, 0, mock(), 0, 0);
+            player.teleportTo(serverLevelSpy, 0, 0, 0, mock(), 0, 0);
 
             // Verify that serverChunkCacheMock.addRegionTicket(...) is called once
             Mockito.verify(serverChunkCacheMock, Mockito.times(1))