image/jpeg: increase TestLargeImageWithShortData timeout by an order of magnitude

Also dump goroutines on failure.

The original bug report in #10413 reported a hang of “several
minutes”. An apparently-spurious failure was observed in
https://build.golang.org/log/e5ac3ce3fb7d04ec13e5bbfadea8bb5869a4dd1e,
with a delay of only 3.64s.

Moreover, if the test does fail due to a regression, we will want a
goroutine dump to diagnose where it got stuck. The current call to
t.Fatalf does not produce such a dump, so is not nearly as useful if
the failure only occasionally reproduces.

Updates #10413.

Change-Id: I6ab9d112f14f438a0c54e02ec95934627acdc64b
Reviewed-on: https://go-review.googlesource.com/c/go/+/408355
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/image/jpeg/reader_test.go

@@ -13,6 +13,7 @@ import (
 	"io"
 	"math/rand"
 	"os"
+	"runtime/debug"
 	"strings"
 	"testing"
 	"time"
@@ -247,18 +248,16 @@ func TestLargeImageWithShortData(t *testing.T) {
 		"\x20\x36\x9f\x78\x64\x75\xe6\xab\x7d\xb2\xde\x29\x70\xd3\x20\x27" +
 		"\xde\xaf\xa4\xf0\xca\x9f\x24\xa8\xdf\x46\xa8\x24\x84\x96\xe3\x77" +
 		"\xf9\x2e\xe0\x0a\x62\x7f\xdf\xd9"
-	c := make(chan error, 1)
-	go func() {
-		_, err := Decode(strings.NewReader(input))
-		c <- err
-	}()
-	select {
-	case err := <-c:
-		if err == nil {
-			t.Fatalf("got nil error, want non-nil")
-		}
-	case <-time.After(3 * time.Second):
-		t.Fatalf("timed out")
+
+	timer := time.AfterFunc(30*time.Second, func() {
+		debug.SetTraceback("all")
+		panic("TestLargeImageWithShortData stuck in Decode")
+	})
+	defer timer.Stop()
+
+	_, err := Decode(strings.NewReader(input))
+	if err == nil {
+		t.Fatalf("got nil error, want non-nil")
 	}
 }