Add workflow_dispatch trigger to GitHub Actions test workflow

This change allows the tests workflow to be manually triggered from the GitHub
UI using the workflow_dispatch event, in addition to the existing push and
pull_request triggers.

Author: krzyzanowskim

.github/workflows/tests.yml

@@ -5,6 +5,7 @@ on:
     branches: [main]
   pull_request:
     branches: [main]
+  workflow_dispatch:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}