ci(tests): run tests on pull requests (#963)

Adds pull_request to trigger workflow when an external contributor creates a pull request from a fork.

.github/workflows/tests.yml

@@ -2,9 +2,10 @@ name: tests
 run-name: Tests
 on:
   workflow_dispatch:
+  pull_request:
   push:
     branches:
-      - "**"
+      - master
 jobs:
   tests-sdk-java:
     if: ${{ !contains(github.event.head_commit.message, '[skip main]') }}