os/exec: set traceback to "system" in TestContextCancel

This will dump more goroutines if the test happens to fail.

For golang#50138.

Change-Id: Ifae30b5ba8bddcdaa9250dd90be8d8ba7d5604d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/442476
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

src/os/exec/exec_test.go

@@ -27,6 +27,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"runtime"
+	"runtime/debug"
 	"strconv"
 	"strings"
 	"sync"
@@ -988,6 +989,7 @@ func TestContextCancel(t *testing.T) {
 		if time.Since(start) > time.Minute {
 			// Panic instead of calling t.Fatal so that we get a goroutine dump.
 			// We want to know exactly what the os/exec goroutines got stuck on.
+			debug.SetTraceback("system")
 			panic("canceling context did not stop program")
 		}