AssumeCulture en-us for month formatting test

• [x ] You've read the Contributor Guide and Code of Conduct.
• [x ] You've included unit or integration tests for your change, where applicable.
• [x ] You've included inline docs for your change, where applicable.
• [x ] There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.

After discussing culture changes in 932, one more place needs to assume culture for the tests to run on a non en-us system.