[async rest] Add coverage for the import error raised within the async rest transport layer.

Add unit test coverage for the following import error within `rest_asyncio.py.` file:

```
try:
import aiohttp # type: ignore
from google.auth.aio.transport.sessions import AsyncAuthorizedSession # type: ignore
from google.api_core import rest_streaming_async # type: ignore
{# NOTE: `pragma: NO COVER` is needed since the coverage for presubmits isn't combined. #}
except ImportError as e: # pragma: NO COVER
raise ImportError("`rest_asyncio` transport requires the library to be installed with the `async_rest` extra. Install the library with the `async_rest` extra using `pip install {{ api.naming.warehouse_package_name }}[async_rest]`") from e
```

We want to make sure the error is raised to the user correctly if they don't have the right deps installed.