CI: Update GHA configuration

Author: amotl

.github/workflows/tests.yml

@@ -1,9 +1,18 @@
 name: Tests
+
 on:
   push:
     branches: [ main ]
-  pull_request:
-    branches: [ main ]
+  pull_request: ~
+
+  # 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 }}