Reduce tests timeout in GitHub Actions

Sometimes a test gets stuck and and we have to wait until it times out
in order to check which files were being tested at the time.

The default timeout is six hours. I'm reducing it to one hour which
should still be plenty of time even on repositories with no knapsack
token.

.github/workflows/tests.yml

@@ -10,6 +10,7 @@ on:
 jobs:
   tests:
     runs-on: ubuntu-18.04
+    timeout-minutes: 60
     services:
       postgres:
         image: postgres:10.10