Do not run tests twice for pull requests

In order to achieve that we're going to run tests only when commits are
pushed to `master` branch (i.e. pull requests get merged) or when pull
requests are created and the target branch is `master`.

Author: ikalnytskyi

.github/workflows/test.yml

@@ -1,8 +1,10 @@
 name: test
 
 on:
-  - push
-  - pull_request
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
 
 jobs:
   lint: