runtime/debug: fix the test

If source are not available, then the stack looks like:
stack_test.go:40: 	/tmp/gobuilder/linux-amd64-race-72b15c5d6f65/go/src/pkg/runtime/debug/bla-bla-bla/src/pkg/runtime/debug/stack_test.go:15 (0x43fb11)
stack_test.go:40: 	/tmp/gobuilder/linux-amd64-race-72b15c5d6f65/go/src/pkg/runtime/debug/bla-bla-bla/src/pkg/runtime/debug/stack_test.go:18 (0x43fb7a)
stack_test.go:40: 	/tmp/gobuilder/linux-amd64-race-72b15c5d6f65/go/src/pkg/runtime/debug/bla-bla-bla/src/pkg/runtime/debug/stack_test.go:37 (0x43fbf4)
stack_test.go:40: 	/tmp/gobuilder/linux-amd64-race-72b15c5d6f65/go/src/pkg/testing/bla-bla-bla/src/pkg/testing/testing.go:301 (0x43b5ba)
stack_test.go:40: 	/tmp/gobuilder/linux-amd64-race-72b15c5d6f65/go/src/pkg/runtime/bla-bla-bla/src/pkg/runtime/proc.c:276 (0x410670)
stack_test.go:40:
which is 6 lines.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6637060

src/pkg/runtime/debug/stack_test.go

@@ -36,7 +36,7 @@ func (t T) method() []byte {
 func TestStack(t *testing.T) {
 	b := T(0).method()
 	lines := strings.Split(string(b), "\n")
-	if len(lines) <= 6 {
+	if len(lines) < 6 {
 		t.Fatal("too few lines")
 	}
 	n := 0