ClassCanBeStatic: Exclude JUnit @Nested classes

From the [JUnit docs](https://junit.org/junit5/docs/current/user-guide/#writing-tests-nested):

> Only non-static nested classes (i.e. inner classes) can serve as
  `@Nested` test classes.

Author: ljrmorgan

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

@@ -43,6 +43,7 @@
     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";
 
@@ -77,6 +78,9 @@ public Description matchClass(ClassTree tree, VisitorState state) {
     if (CanBeStaticAnalyzer.referencesOuter(tree, currentClass, state)) {
       return NO_MATCH;
     }
+    if (hasAnnotation(currentClass, JUNIT_NESTED_ANNOTATION, state)) {
+      return NO_MATCH;
+    }
     if (tree.getMembers().stream().anyMatch(m -> hasAnnotation(m, REFASTER_ANNOTATION, state))) {
       return NO_MATCH;
     }

core/src/test/java/com/google/errorprone/bugpatterns/ClassCanBeStaticTest.java

@@ -401,4 +401,28 @@ public void refaster() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void junitNestedClass() {
+    compilationHelper
+        .addSourceLines(
+            "Nested.java",
+            "package org.junit.jupiter.api;",
+            "import java.lang.annotation.ElementType;",
+            "import java.lang.annotation.Retention;",
+            "import java.lang.annotation.RetentionPolicy;",
+            "import java.lang.annotation.Target;",
+            "@Target(ElementType.TYPE)",
+            "@Retention(RetentionPolicy.RUNTIME)",
+            "public @interface Nested {}")
+        .addSourceLines(
+            "A.java",
+            "import org.junit.jupiter.api.Nested;",
+            "public class A {",
+            "  @Nested class Inner {",
+            "    void f() {}",
+            "  }",
+            "}")
+        .doTest();
+  }
 }