cmd/cover: fix test on Solaris
cmd/cover: fix test on Solaris

Solaris diff doesn't know about -d (a.k.a. --minimal).

Change-Id: I86146b4b93e3d0bdea52a0fde59afca20b5bab51
Reviewed-on: https://go-review.googlesource.com/117335
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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