Fix test suite failure if filter_pdf_page is missing

src/ocrmypdf/builtin_plugins/default_filters.py

@@ -0,0 +1,12 @@
+# © 2021 James R. Barlow: github.com/jbarlow83
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+from ocrmypdf import hookimpl
+
+
+@hookimpl
+def filter_pdf_page(page, image_filename, output_pdf):
+    return output_pdf