cmd/go: skip external tests on plan9/arm

CL 13166, CL 13342 and CL 33425 skipped external tests
on freebsd/arm, linux/arm and linux/mips.

This CL does the same for plan9/arm to reduce test time
on plan9/arm and prevent the Go builder to time out.

Change-Id: I16fcc5d8010a354f480673b8c4a8a11dbc833557
Reviewed-on: https://go-review.googlesource.com/90416
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -81,6 +81,13 @@ func init() {
 			skipExternal = true
 			canRun = false
 		}
+	case "plan9":
+		switch runtime.GOARCH {
+		case "arm":
+			// many plan9/arm machines are too slow to run
+			// the full set of external tests.
+			skipExternal = true
+		}
 	case "windows":
 		exeSuffix = ".exe"
 	}