test/ruby/test_enumerator.rb: check the deprecation warning

by explicitly setting `Warning[:deprecated] = true`.
I removed "capture_io" at 79063d8cbf, but
it printed the warning when `RUBYOPT=-w`.

This change makes the warnings enabled explicitly, capture and check the
warning.

test/ruby/test_enumerator.rb

@@ -69,7 +69,15 @@ class TestEnumerator < Test::Unit::TestCase
 
   def test_initialize
     assert_equal([1, 2, 3], @obj.to_enum(:foo, 1, 2, 3).to_a)
-    assert_equal([1, 2, 3], Enumerator.new(@obj, :foo, 1, 2, 3).to_a)
+    begin
+      deprecated_bak, Warning[:deprecated] = Warning[:deprecated], true
+      _, err = capture_io do
+        assert_equal([1, 2, 3], Enumerator.new(@obj, :foo, 1, 2, 3).to_a)
+      end
+      assert_match 'Enumerator.new without a block is deprecated', err
+    ensure
+      Warning[:deprecated] = deprecated_bak
+    end
     assert_equal([1, 2, 3], Enumerator.new { |y| i = 0; loop { y << (i += 1) } }.take(3))
     assert_raise(ArgumentError) { Enumerator.new }