ci: also fire tests on main (default branch is main, not master)

The original tests.yml triggered only on push to master and PRs into
master. Pushing this branch and opening PR #232 against main produced
'no checks reported' because the workflow has no matching trigger.

Add main to both branches: arrays. Keep master as well so the workflow
keeps firing on stale branches that may still receive commits.

Author: vietanhdev

.github/workflows/tests.yml

@@ -2,10 +2,10 @@ name: Tests
 
 on:
   push:
-    branches: [master]
+    branches: [main, master]
     tags: ['v*']
   pull_request:
-    branches: [master]
+    branches: [main, master]
   workflow_call:
 
 jobs: