Only run tests against pushes to 'main'.

Author: jyasskin

.github/workflows/tests.yml

@@ -1,7 +1,8 @@
 name: Tests
 on:
-  - push
-  - pull_request
+  push:
+    branches: [main]
+  pull_request:
 defaults:
   run:
     working-directory: webserver