CI: Improve GHA workflow file

Author: amotl

.github/workflows/tests.yml

@@ -5,6 +5,15 @@ on:
     branches: [ master ]
   pull_request:
     branches: [ master ]
+
+  # 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:
     runs-on: ${{ matrix.os }}