cmd/cover: fix off-by-one error in TestCoverHTML
cmd/cover: fix off-by-one error in TestCoverHTML

Avoid index out of range if len(goldenLines) == len(outLines) + 1

Change-Id: Ic23a85d2b8dd06a615e35a58331e78abe4ad6703
Reviewed-on: https://go-review.googlesource.com/130396
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,akutz/go,CAFxX/go,akutz/go,akutz/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,josharian/go,akutz/go,nawawi/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,golang/go,golang/go,josharian/go,golang/go,akutz/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,CAFxX/go,golang/go,josharian/go,golang/go,golang/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go