Add spec for Warning.categories

Author: herwinw

core/warning/categories_spec.rb

@@ -0,0 +1,9 @@
+require_relative '../../spec_helper'
+
+ruby_version_is "3.4" do
+  describe "Warning.categories" do
+    it "returns the list of possible warning categories" do
+      Warning.categories.sort.should == [:deprecated, :experimental, :performance, :strict_unused_block]
+    end
+  end
+end