test: suppress unused variable warning in orphan test

Author: themechbro

core/src/test/java/io/grpc/internal/ManagedChannelOrphanWrapperTest.java

@@ -148,6 +148,7 @@ public void orphanedChannel_triggerWarningAndCoverage() {
         new ConcurrentHashMap<>();
 
     // Create the wrapper but NEVER call shutdown
+    @SuppressWarnings("UnusedVariable")
     ManagedChannelOrphanWrapper wrapper = new ManagedChannelOrphanWrapper(mc, refqueue, refs);
     wrapper = null; // Make it eligible for GC