tests: extend cmd/desc integration test

[eirinikos]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

---

This change adds tests asserting that brew desc returns the expected output when run with one flag, with more than one flag, and without an argument.

If anyone has suggestions for improving the code or the formatting, let me know, thanks!

[MikeMcQuaid]

Looks great, nice work 👍