Fix test failure in `no-default-features` (#2450)

* Fix test failure in `no-default-features`

This commit fixes a test failure from `test_clients_from_sdk_config`. It
is a negative test expecting a panic, but it failed because it panicked
for a different reason, i.e. "no default sleep implementation available."

* Avoid bringing in `rt-tokio` for `aws-smithy-async`

This commit addresses https://github.com/awslabs/smithy-rs/pull/2450#discussion_r1134426351

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Co-authored-by: Yuki Saito <awsaito@amazon.com>