ci: ensures tests are run on push

Unit tests job is currently only executed on PRs, leaving pushes on e.g.
incubation branch untested. This can lead to issues like the one
described in opendatahub-io#1235

This PR enables GH Action to be run also for push events.

.github/workflows/unit-tests.yaml

@@ -1,6 +1,5 @@
 name: Unit Tests
-on:
-  pull_request:
+on: [push, pull_request]
 jobs:
   unit-test:
     runs-on: ubuntu-latest