Part of #1971384: [META] Convert page callbacks to controllers
For instructions on how to convert a page callback into a controller, see the WSCCI Conversion Guide.
Need to rewrite the whole module to make test sync with current test implementation. For more details, please refer: #1988802: [META] Rewrite test modules in system to provide better unit testing.
This appears to be already done in commit 22df596! :D
The commit mentioned in #4 isolated ajax_test_dialog_contents(), but it is not removing it. Still we have a controller method that calls ajax_test_dialog_contents().
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