Fix ThreadSanitizer data race and optimize AsyncWrapperTest

Author: tejasiyer-dev

sdks/java/core/src/test/java/org/apache/beam/sdk/transforms/AsyncWrapperTest.java

@@ -179,7 +179,7 @@ public BagState<T> readLater() {
 
   // 4. Used for testing Timer mock implementations.
   private static class FakeTimer implements Timer {
-    private Instant time = Instant.EPOCH;
+    private volatile Instant time = Instant.EPOCH;
 
     @Override
     public void set(Instant absoluteTime) {