os/signal: check MustHaveExec in TestAtomicStop

Change-Id: I284ecfab574b3058ba66b75f8f8d2ccdb90a8a0c
Reviewed-on: https://go-review.googlesource.com/46650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/signal/signal_test.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"flag"
 	"fmt"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -313,6 +314,8 @@ func TestAtomicStop(t *testing.T) {
 		t.Fatal("atomicStopTestProgram returned")
 	}
 
+	testenv.MustHaveExec(t)
+
 	const execs = 10
 	for i := 0; i < execs; i++ {
 		cmd := exec.Command(os.Args[0], "-test.run=TestAtomicStop")