github: workflows: Process unit tests on 4.2 branch

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

.github/workflows/unit-tests.yaml

@@ -22,6 +22,7 @@ on:
       - 'examples/**'
     branches:
       - master
+      - 4.2
       - 4.1
       - 4.0
       - 3.2