Redirect output of musttail attribute test

Checklist:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to [Notes][notes] and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] 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).

cc @TerrorJack