[ruby/digest] Use pend instead of skip

https://github.com/ruby/digest/commit/82fb618157

test/digest/test_ractor.rb

@@ -14,7 +14,7 @@ module TestDigestRactor
   Data2 = "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"
 
   def setup
-    skip unless defined?(Ractor)
+    pend unless defined?(Ractor)
   end
 
   def test_s_hexdigest