cmd/go/internal/lockedfile/internal/filelock: fix test on iOS

Change-Id: I0390b382db0ca36de20af0ef15204c5bfc084d3d
Reviewed-on: https://go-review.googlesource.com/c/151937
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/lockedfile/internal/filelock/filelock_test.go

@@ -8,6 +8,7 @@ package filelock_test
 
 import (
 	"fmt"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -183,6 +184,8 @@ func TestRLockExcludesOnlyLock(t *testing.T) {
 }
 
 func TestLockNotDroppedByExecCommand(t *testing.T) {
+	testenv.MustHaveExec(t)
+
 	f, remove := mustTempFile(t)
 	defer remove()