add unit tests for NilClass

test/t/nil.rb

@@ -5,6 +5,15 @@ assert('NilClass', '15.2.4') do
   assert_equal Class, NilClass.class
 end
 
+assert('NilClass', '15.2.4.1') do
+  assert_equal NilClass, nil.class
+  assert_false NilClass.method_defined? :new
+end
+
+assert('NilClass superclass', '15.2.4.2') do
+  assert_equal Object, NilClass.superclass
+end
+
 assert('NilClass#&', '15.2.4.3.1') do
   assert_false nil.&(true)
   assert_false nil.&(nil)