add test cases for superfluous days

[TymekDev]

This PR adds cases for whole years and whole months to not have extra days, e.g. May (2023-05-11T00:00:00.00) to July (2023-07-11T00:00:00.00) being 2 months instead of current 2 months, 1 day.

Related issue: #262

[primer-css]

👋 Hello and thanks for pinging us! This issue or PR has been added to our inbox and a Design Infrastructure first responder will review it soon.

• 🎨 If this is a PR that includes a visual change, please make sure to add screenshots in the description or deploy this code to a lab machine with instructions for how to test.
• ⏩ If this is a PR that includes changes to an interaction, please include a video recording in the description.
• ⚠️ If this is urgent, please visit us in #primer on Slack and tag the first responders listed in the channel topic.

[lukasoppermann]

I'd love for @keithamus to approve this as he is working on it. Fine from primer FR side

[keithamus]

Thanks, I'm looking into fixing this but I have higher priorities for now. A fix will come though!