CI: Bump timeout from 3h to 4h. shmem-tests cause debug test runs to take longer.

After addition of new large_inserts_stress_test, being done as part of
PR #569 (free/memory mgmt support for shared memory), CI-debug jobs
are timing out at current timeout=3h.

Bump this timeout limit to 4h, to see if test-jobs complete.

Author: gapisback

ci/steps.lib.yml

@@ -11,7 +11,7 @@
 #@   elif test_nightly:
 #@     return "6h"
 #@   else:
-#@     return "3h"
+#@     return "4h"
 #@   end
 #@ end