Fix tautological assertions in test_each_value in test_hash.rb

Both assertions tested `[].length == 0` rather than `res.length`,
so the test passed even when `each_value` yielded nothing.

Author: sampokuokkanen

test/ruby/test_hash.rb

@@ -465,10 +465,10 @@ def test_each_pair
   def test_each_value
     res = []
     @cls[].each_value { |v| res << v }
-    assert_equal(0, [].length)
+    assert_equal(0, res.length)
 
     @h.each_value { |v| res << v }
-    assert_equal(0, [].length)
+    assert_equal(@h.size, res.length)
 
     expected = []
     @h.each { |k, v| expected << v }