cmd/cover: fix TestCoverHTML on Plan 9

cmd/cover: fix TestCoverHTML on Plan 9

CL 116975 added TestCoverHTML. However, this test is failing
on Plan 9, because the GNU diff tool is called "ape/diff"
instead of "diff" on Plan 9.

This change replaces the "diff" command by the "ape/diff"
command on Plan 9.

Fixes #25795.

Change-Id: I15b49868cd09f3f977aa13fffdfc430c882bf757
Reviewed-on: https://go-review.googlesource.com/117415
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>