rewrite "it" statements in tests to use "shall" instead of "should"

as per recommended practice for writing good requirements; "shall" is verifiable, "should" is not.

## references
- https://www.nasa.gov/reference/appendix-c-how-to-write-a-good-requirement/#hds-sidebar-nav-1
- https://argondigital.com/blog/product-management/using-the-correct-terms-shall-will-should/