Fix mock_client macro (#4013)

## Motivation and Context
There was downstream breakage (the macro was always broken, but #4001
made it more likely to happen)

## Description
- Use `$crate` to fix the issue when `mock_client` was not otherwise in
scope
- Add a test
## Testing
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## Checklist
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rather than leaving it unchecked -->
- [ ] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [ ] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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Co-authored-by: Landon James <lnj@amazon.com>