[ci] Run test workflow nightly, enable manual trigger (#629)

At least for now I think it can be useful to trigger the test pipeline
automatically to spot any problems. I also propose to
add `workflow_dispatch` which allows us to manually trigger a run via
the GitHub UI without having to push a commit.

.github/workflows/test.yml

@@ -7,6 +7,9 @@ on:
   push:
     branches:
     - master
+  workflow_dispatch:
+  schedule:
+    - cron: '0 4 * * *'
 
 env:
   PIP_DISABLE_PIP_VERSION_CHECK: '1'