Add regression test for Bug #20573

Just a regression test to ensure behavior remains the same

test/ruby/test_warning.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require 'test/unit'
+
+class TestWarning < Test::Unit::TestCase
+  def test_warn_called_only_when_category_enabled
+    # Assert that warn is called when the category is enabled
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+      begin;
+        Warning[:deprecated] = true
+        $warnings = []
+        def Warning.warn(msg, category:)
+          $warnings << [msg, category]
+        end
+        assert_equal(0, $warnings.length)
+        "" << 12
+        assert_equal(1, $warnings.length)
+      end;
+
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+      begin;
+        Warning[:deprecated] = false
+        $warnings = []
+        def Warning.warn(msg, category:)
+          $warnings << [msg, category]
+        end
+        assert_equal(0, $warnings.length)
+        "" << 12
+        assert_equal(0, $warnings.length, $warnings.join)
+      end;
+  end
+end