os/exec: re-enable TestExtraFiles for plan9

This test should now succeed after CL 22610 which fixes issue #7118

Change-Id: Ie785a84d77b27c832a1ddd81699bf25dab24b97d
Reviewed-on: https://go-review.googlesource.com/22640Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/exec/exec_test.go

@@ -660,10 +660,6 @@ func TestHelperProcess(*testing.T) {
 			// the cloned file descriptors that result from opening
 			// /dev/urandom.
 			// https://golang.org/issue/3955
-		case "plan9":
-			// TODO(0intro): Determine why Plan 9 is leaking
-			// file descriptors.
-			// https://golang.org/issue/7118
 		case "solaris":
 			// TODO(aram): This fails on Solaris because libc opens
 			// its own files, as it sees fit. Darwin does the same,