Run unittest tests with unittest

The unit tests in the following directories are written to unittest, not
pytest:

* `samples/client/echo_api/python/`
* `samples/client/echo_api/python-disallowAdditionalPropertiesIfNotPresent/`

If pytest is told to execute these tests, it will log warnings about
being unable to collect certain tests.

Author: jeremy-audet

.github/workflows/samples-python-client-echo-api.yaml

@@ -42,7 +42,7 @@ jobs:
 
       - name: Test
         working-directory: ${{ matrix.sample }}
-        run: python -m pytest
+        run: python -m unittest discover
 
       - name: mypy
         working-directory: ${{ matrix.sample }}