update gha to run unit tests in PRs

Author: l-trotta

.github/workflows/unit-tests.yml

@@ -1,6 +1,11 @@
 name: Java Unit Tests
 
 on:
+  pull_request:
+    branches:
+      - main
+      - '[0-9]+.[0-9]+'
+    types: [ opened, synchronize, reopened ]
   workflow_run:
     workflows: ['Validate Gradle Wrapper']
     types: [completed]