refactor(tests): testing.util.async_request to client.AsyncHTTPClient

[sanspareilsmyn]

What does this PR address?

Fixes #3411

• The async_request function in bento.testing.utils performs an action that duplicates the functionality of AsyncHTTPClient in bento.client. Since the Client is correctly implemented, I have refactored the test code to operate using it.
• Since AsyncHTTPClient use httpx, there's minor datatype change on aiohttp.FormData.

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[frostming]

Awesome, thanks!