runtime: use GOTRACEBACK=system for TestStackBarrierProfiling

This should help with debugging failures.

For #15138 and #15477.

Change-Id: I77db2b6375d8b4403d3edf5527899d076291e02c
Reviewed-on: https://go-review.googlesource.com/23134
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/pprof/pprof_test.go

@@ -388,7 +388,7 @@ func TestStackBarrierProfiling(t *testing.T) {
 			args = append(args, "-test.short")
 		}
 		cmd := exec.Command(os.Args[0], args...)
-		cmd.Env = append([]string{"GODEBUG=gcstackbarrierall=1", "GOGC=1"}, os.Environ()...)
+		cmd.Env = append([]string{"GODEBUG=gcstackbarrierall=1", "GOGC=1", "GOTRACEBACK=system"}, os.Environ()...)
 		if out, err := cmd.CombinedOutput(); err != nil {
 			t.Fatalf("subprocess failed with %v:\n%s", err, out)
 		}