Revert "[ruby/securerandom] Use build_message instead of message for test-unit"

This reverts commit 7f9f5f5723.

test/test_securerandom.rb

@@ -175,7 +175,7 @@ end
   end
 
   def assert_in_range(range, result, mesg = nil)
-    assert(range.cover?(result), build_message(mesg) {"Expected #{result} to be in #{range}"})
+    assert(range.cover?(result), message(mesg) {"Expected #{result} to be in #{range}"})
   end
 
   def test_with_openssl