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>

src/cmd/cover/cover_test.go

@@ -310,8 +310,12 @@ func TestCoverHTML(t *testing.T) {
 	if err := ioutil.WriteFile(htmlHTML, out.Bytes(), 0644); err != nil {
 		t.Fatal(err)
 	}
+	diff := "diff"
+	if runtime.GOOS == "plan9" {
+		diff = "/bin/ape/diff"
+	}
 	// diff -uw testdata/html/html.html testdata/html/html.golden
-	cmd = exec.Command("diff", "-u", "-w", htmlHTML, htmlGolden)
+	cmd = exec.Command(diff, "-u", "-w", htmlHTML, htmlGolden)
 	run(cmd, t)
 }