Suppress a warning from Error Prone. (#2724)

These inner classes are deliberately not static for the purposes of tests.

gson/src/test/java/com/google/gson/reflect/TypeTokenTest.java

@@ -282,6 +282,7 @@ public void testParameterizedFactory_Invalid() {
                 + GenericWithMultiBound.class);
 
     class Outer {
+      @SuppressWarnings("ClassCanBeStatic")
       class NonStaticInner<T> {}
     }
 
@@ -344,6 +345,7 @@ private static <M> void createTypeTokenTypeVariable() {
   public void testTypeTokenTypeVariable() throws Exception {
     // Put the test code inside generic class to be able to access `T`
     class Enclosing<T> {
+      @SuppressWarnings("ClassCanBeStatic")
       class Inner {}
 
       void test() {