fix: Async system tests were not unwrapping async_generators (#1086)

Currently the system tests were failing with a message "TypeError:
'async_generator' object is not callable".

This is because newer versions of pytest_asyncio uses "strict" mode
by default which does not automatically convert fixtures to async
fixtures in async test cases. This became an issue as of pytest_asyncio
version 19.

The solution is to either switch the mode to "auto" in the nox
commandline or to mark the async fixtures as async fixtures.

I've opted for the second to avoid making this hard to discover.

system_tests/system_tests_async/conftest.py

@@ -19,6 +19,7 @@
 import google.auth.transport.requests
 import google.auth.transport.urllib3
 import pytest
+import pytest_asyncio
 import requests
 import urllib3
 
@@ -30,37 +31,37 @@
 TOKEN_INFO_URL = "https://www.googleapis.com/oauth2/v3/tokeninfo"
 
 
-@pytest.fixture
+@pytest_asyncio.fixture
 def service_account_file():
     """The full path to a valid service account key file."""
     yield sync_conftest.SERVICE_ACCOUNT_FILE
 
 
-@pytest.fixture
+@pytest_asyncio.fixture
 def impersonated_service_account_file():
     """The full path to a valid service account key file."""
     yield sync_conftest.IMPERSONATED_SERVICE_ACCOUNT_FILE
 
 
-@pytest.fixture
+@pytest_asyncio.fixture
 def authorized_user_file():
     """The full path to a valid authorized user file."""
     yield sync_conftest.AUTHORIZED_USER_FILE
 
 
-@pytest.fixture
+@pytest_asyncio.fixture
 async def aiohttp_session():
     async with aiohttp.ClientSession(auto_decompress=False) as session:
         yield session
 
 
-@pytest.fixture(params=["aiohttp"])
+@pytest_asyncio.fixture(params=["aiohttp"])
 async def http_request(request, aiohttp_session):
     """A transport.request object."""
     yield aiohttp_requests.Request(aiohttp_session)
 
 
-@pytest.fixture
+@pytest_asyncio.fixture
 async def token_info(http_request):
     """Returns a function that obtains OAuth2 token info."""
 
@@ -85,7 +86,7 @@ async def _token_info(access_token=None, id_token=None):
     yield _token_info
 
 
-@pytest.fixture
+@pytest_asyncio.fixture
 async def verify_refresh(http_request):
     """Returns a function that verifies that credentials can be refreshed."""