Enable GH Actions tests to be manually triggered

https://docs.github.com/en/actions/managing-workflow-runs/manually-running-a-workflow

Author: ericyhwang

.github/workflows/test.yml

@@ -7,6 +7,10 @@ on:
   pull_request:
     branches:
     - master
+  # Allow manually triggering tests
+  workflow_dispatch:
+    branches:
+    - master
 
 jobs:
   test: