cmd/cover: add test for HTML output

This adds a case for what was fixed in 4fe688c6 to prevent regression;
a follow-on change will address #25767.

Change-Id: Iced8cc10e2993ef7caf7e9c59ffbc7147d78ddd7
Reviewed-on: https://go-review.googlesource.com/116975


Run-TryBot: David Symonds <dsymonds@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>