Discourage use of JUnit3 via static analysis.

Putting this in `JdkObsolete` is a bit of a hack, but we already have non-JDK APIs in there (`org.apache.commons.io.IOUtils`), so...

PiperOrigin-RevId: 908153869

Author: kluever

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

@@ -152,7 +152,8 @@ Optional<Fix> fix(Tree tree, VisitorState state) {
                   "Dictionary is a nonstandard class that predates the Java Collections Framework;"
                       + " use LinkedHashMap."),
               new Obsolete(
-                  "java.util.Enumeration", "Enumeration is an ancient precursor to Iterator."))
+                  "java.util.Enumeration", "Enumeration is an ancient precursor to Iterator."),
+              new Obsolete("junit.framework.TestCase", "JUnit3 is obsolete; use JUnit4 instead."))
           .stream()
           .collect(toImmutableMap(Obsolete::qualifiedName, x -> x));