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

[ohmayr]

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.