[Python] Improve streaming Blob testing suite

[crisidev]

The Blob tests are far from ideal. They do not work with asyncio is involved and it seems like during tests the Blob is loaded in memory and not really streamed, while when running on a real application the streaming feature seems to be working properly.

https://github.com/awslabs/smithy-rs/blob/main/rust-runtime/aws-smithy-http-server-python/src/types.rs#L499

[unexge]

I believe this is addressed with #2431. I didn't pay attention to the issue about loading the whole content to the memory, but it seems like the test is problematic (I think we should use stream::repeat instead of String::repeat). I'll close this ticket, but we can revisit that problem as part of #1752