Add a failing test for https://bugs.ruby-lang.org/issues/20305.

This bug demonstrates the issue I reported. It passes on commit
114e71d
but does not pass on commit (the immediate child of the above commit)
1d2d25d

test/ruby/test_string.rb

@@ -1130,6 +1130,11 @@ def test_grapheme_clusters_memory_leak
     end;
   end
 
+  def test_byteslice_grapheme_clusters
+    string = "안녕"
+    assert_equal(["안"], string.byteslice(0,4).grapheme_clusters)
+  end
+
   def test_each_line
     verbose, $VERBOSE = $VERBOSE, nil