ci: twister: add unit tests for pytest plugin

Adding a stage to twister unit test workflow where tests for
pytest-twister-harness plugin are executed.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>

.github/workflows/twister_tests.yml

@@ -49,10 +49,18 @@ jobs:
     - name: install-packages
       run: |
         pip3 install -r scripts/requirements-base.txt -r scripts/requirements-build-test.txt
-    - name: Run pytest
+    - name: Run pytest for twisterlib
       env:
         ZEPHYR_BASE: ./
         ZEPHYR_TOOLCHAIN_VARIANT: zephyr
       run: |
         echo "Run twister tests"
         PYTHONPATH=./scripts/tests pytest ./scripts/tests/twister
+    - name: Run pytest for pytest-twister-harness
+      env:
+        ZEPHYR_BASE: ./
+        ZEPHYR_TOOLCHAIN_VARIANT: zephyr
+        PYTHONPATH: ./scripts/pylib/pytest-twister-harness/src:${PYTHONPATH}
+      run: |
+        echo "Run twister tests"
+        pytest ./scripts/pylib/pytest-twister-harness/tests