Increase the timeout value of Libcore tests for gcstress non-debug configs.

Increase the per-test timeout value passed to Vogar by script
`tools/run-libcore-tests.sh` for gcstress non-debug configurations
from 900 seconds (15 minutes) to 1200 seconds (20 minutes), to allow
long Libcore tests (like `libcore.java.lang`) to finish on slower
devices like bonito (Pixel 3a XL).

Test: Rely on the ART Buildbot (LUCI)
Bug: 158195843
Change-Id: I9353d8a072ddfcbdb01cb4eba228fbcdd01596ee
diff --git a/tools/run-libcore-tests.sh b/tools/run-libcore-tests.sh
index 31beeb5..26ba9ee 100755
--- a/tools/run-libcore-tests.sh
+++ b/tools/run-libcore-tests.sh
@@ -269,7 +269,7 @@
     if $debug; then
       default_timeout_secs=1440
     else
-      default_timeout_secs=900
+      default_timeout_secs=1200
     fi
   elif $heap_poisoning && $debug; then
     # Increase the timeout for heap poisoning and debug combo