ClassCanBeStatic: Ignore if annotated with @Keep

Replace the check for the `@Nested` annotation with a check for `@Keep`,
and consider `@Nested` to be `@Keep`-annotated.

Author: ljrmorgan

check_api/src/main/java/com/google/errorprone/util/ASTHelpers.java

@@ -1041,7 +1041,9 @@ public static boolean shouldKeep(Tree tree) {
    * though they are.
    */
   private static final ImmutableSet<String> ANNOTATIONS_CONSIDERED_KEEP =
-      ImmutableSet.of("org.apache.beam.sdk.transforms.DoFn.ProcessElement");
+      ImmutableSet.of(
+          "org.apache.beam.sdk.transforms.DoFn.ProcessElement",
+          "org.junit.jupiter.api.Nested");
 
   private static final String USED_REFLECTIVELY = "UsedReflectively";
 

core/src/main/java/com/google/errorprone/bugpatterns/ClassCanBeStatic.java

@@ -43,7 +43,6 @@
     tags = {StandardTags.STYLE, StandardTags.PERFORMANCE})
 public class ClassCanBeStatic extends BugChecker implements ClassTreeMatcher {
 
-  private static final String JUNIT_NESTED_ANNOTATION = "org.junit.jupiter.api.Nested";
   private static final String REFASTER_ANNOTATION =
       "com.google.errorprone.refaster.annotation.BeforeTemplate";
 
@@ -78,7 +77,7 @@ public Description matchClass(ClassTree tree, VisitorState state) {
     if (CanBeStaticAnalyzer.referencesOuter(tree, currentClass, state)) {
       return NO_MATCH;
     }
-    if (hasAnnotation(currentClass, JUNIT_NESTED_ANNOTATION, state)) {
+    if (ASTHelpers.shouldKeep(tree)) {
       return NO_MATCH;
     }
     if (tree.getMembers().stream().anyMatch(m -> hasAnnotation(m, REFASTER_ANNOTATION, state))) {