Add a unit test that captures the current behavior of formatting of char types and char-backed enum types through StrCat(), StrFormat("%v"), and Substitute().

This test allows us to modify the behavior in this space without introducing undesired changes elsewhere.

PiperOrigin-RevId: 537981963
Change-Id: Icda91b66efcc0dc8c263011b137e130a3db2dc19