[tests only] Add keepalive-workflow (#3)

Author: rfay

.github/workflows/tests.yml

@@ -76,3 +76,6 @@ jobs:
     - name: tests
       run: bats tests
 
+      # keepalive-workflow adds a dummy commit if there's no other action here, keeps
+      # GitHub from turning off tests after 60 days
+    - uses: gautamkrishnar/keepalive-workflow@v1