Fix pytest fixture deprecation warning (#698)

Gets rid of this error:
```
tests/conftest.py:109
  /Users/wkillian/Desktop/NeMo-Agent-Toolkit/tests/conftest.py:109: PytestRemovedIn9Warning: Marks applied to fixtures have no effect
  See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function
    @pytest.mark.usefixtures("restore_environ")
```

Closes

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- I am familiar with the [Contributing Guidelines](https://github.com/NVIDIA/NeMo-Agent-Toolkit/blob/develop/docs/source/resources/contributing.md).
- We require that all contributors "sign-off" on their commits. This certifies that the contribution is your original work, or you have rights to submit it under the same license, or a compatible license.
  - Any contribution which contains commits that are not Signed-Off will not be accepted.
- When the PR is ready for review, new or existing tests cover these changes.
- When the PR is ready for review, the documentation is up to date with these changes.

Authors:
  - Will Killian (https://github.com/willkill07)

Approvers:
  - Eric Evans II (https://github.com/ericevans-nv)

URL: #698

Author: willkill07

tests/conftest.py

@@ -106,9 +106,8 @@ def restore_environ_fixture():
             del (os.environ[key])
 
 
-@pytest.mark.usefixtures("restore_environ")
 @pytest.fixture(name="set_test_api_keys")
-def set_test_api_keys_fixture():
+def set_test_api_keys_fixture(restore_environ):  # pylint: disable=unused-argument
     for key in ("NGC_API_KEY", "NVD_API_KEY", "NVIDIA_API_KEY", "OPENAI_API_KEY", "SERPAPI_API_KEY"):
         os.environ[key] = "test_key"