misc/cgo/testsanitizers: report stderr if go run tsan*.go fails

Update #14559.

Change-Id: I8894f8e16902685f55a21d902199fbe0905f0558
Reviewed-on: https://go-review.googlesource.com/20050
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

misc/cgo/testsanitizers/test.bash

@@ -103,6 +103,7 @@ if test "$tsan" = "yes"; then
     err=${TMPDIR}/tsanerr$$.out
 
     if ! go run tsan.go 2>$err; then
+	cat $err
 	echo "FAIL: tsan"
 	status=1
     elif grep -i warning $err >/dev/null 2>&1; then
@@ -112,6 +113,7 @@ if test "$tsan" = "yes"; then
     fi
 
     if ! go run tsan2.go 2>$err; then
+	cat $err
 	echo "FAIL: tsan2"
 	status=1
     elif grep -i warning $err >/dev/null 2>&1; then