tests/unit_tests: fix tests broken by fastapi upgrade

The `fastapi` version upgrade affected unit tests
as `TestClient` has been upgraded to use `httpx`.
Reference to the release note:
https://fastapi.tiangolo.com/release-notes/#0870
This introduced the below failure in the API unit
tests:
```
>           raise RuntimeError('Event loop is closed')
E           RuntimeError: Event loop is closed

/usr/local/lib/python3.11/asyncio/base_events.py:520: RuntimeError
```
Based on the observation, it seems like somehow
`TestClient` is closing event loop after executing the
first test.
It is backed by the GH issue:
encode/starlette#2069
Fix the issue by using the same test client for all the
synchronous tests.
Fixture `event_loop` is not required anymore after changing
`TestClient` scope to `session`. Hence, drop it.

Signed-off-by: Jeny Sadadia <jeny.sadadia@collabora.com>

Author: Jeny Sadadia

tests/unit_tests/conftest.py

@@ -11,7 +11,6 @@
 """pytest fixtures for KernelCI API"""
 
 from unittest.mock import AsyncMock
-import asyncio
 import fakeredis.aioredis
 from fastapi.testclient import TestClient
 from fastapi import Request, HTTPException, status
@@ -96,12 +95,12 @@ def mock_get_current_admin_user(request: Request):
 app.dependency_overrides[get_current_superuser] = mock_get_current_admin_user
 
 
-@pytest.fixture
+@pytest.fixture(scope='session')
 def test_client():
     """Fixture to get FastAPI Test client instance"""
     # Mock dependency callables for getting current user
     with TestClient(app=versioned_app, base_url=BASE_URL) as client:
-        return client
+        yield client
 
 
 @pytest.fixture
@@ -113,20 +112,6 @@ async def test_async_client():
         await versioned_app.router.shutdown()
 
 
-@pytest.fixture
-def event_loop():
-    """Create an instance of the default event loop for each test case.
-    This is a workaround to prevent the default event loop to be closed by
-    async pubsub tests. It was causing other tests unable to run.
-    The issue has already been reported here:
-    https://github.com/pytest-dev/pytest-asyncio/issues/371
-    """
-    loop = asyncio.new_event_loop()
-    asyncio.set_event_loop(loop)
-    yield loop
-    loop.close()
-
-
 @pytest.fixture
 def mock_db_create(mocker):
     """Mocks async call to Database class method used to create object"""