Change test of TreeSet into test of HashSet (#1328)

Also adds back the TreeSet test as a (skipped) test for #1330.

checker/tests/nullness/jdkannotations/HashtableTest.java

@@ -0,0 +1,20 @@
+import java.util.Hashtable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+class HashtableTest {
+
+    public static void main(String[] args) {
+
+        //:: error: (type.argument.type.incompatible)
+        Hashtable<@Nullable Integer, String> ht1 = new Hashtable<>();
+
+        // Suffers null pointer exception
+        ht1.put(null, "hello");
+
+        //:: error: (type.argument.type.incompatible)
+        Hashtable<Integer, @Nullable String> ht2 = new Hashtable<>();
+
+        // Suffers null pointer exception
+        ht2.put(42, null);
+    }
+}

checker/tests/nullness/jdkannotations/TreeSetTest.java

@@ -1,3 +1,8 @@
+// Partial test case for issue #1330: https://github.com/typetools/checker-framework/issues/1330
+// This should be expanded to include all the cases in the issue.
+
+// @skip-test until we fix the issue
+
 import java.util.TreeSet;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
@@ -8,6 +13,7 @@ public static void main(String[] args) {
         //:: error: (type.argument.type.incompatible)
         TreeSet<@Nullable Integer> ts = new TreeSet<>();
 
+        // This throws a null pointer exception
         ts.add(null);
     }
 }