Redirect output of musttail attribute test

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Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• Configure output was a little wonky -- the compilation output for the test that checked whether the attribute existed leaked to the user's console. This simple fix will redirect it to /dev/null (like all other checks).

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.