CI: Improve GHA job configuration to save resources on redundant builds

Author: amotl

Diff for: .github/workflows/tests.yml

@@ -6,6 +6,14 @@ on:
   pull_request:
     branches: [ main ]
 
+  # Allow job to be triggered manually.
+  workflow_dispatch:
+
+# Cancel in-progress jobs when pushing to the same branch.
+concurrency:
+  cancel-in-progress: true
+  group: ${{ github.workflow }}-${{ github.ref }}
+
 jobs:
 
   tests: