In func TestXxxx(*testing.T) the Xxxx can be anything that can appear
in an identifier, but can't start with a lowercase letter. Clarify the docs.

Fixes #23322

Change-Id: I5c297916981f7e3890ee955d12bc7422a75488e2
Reviewed-on: https://go-review.googlesource.com/86001Reviewed-by: Rob Pike <r@golang.org>