Fix flaky test in aws-sigv4 (#3990)

## Motivation and Context
The test was flaky because all invalid headers are counted as rejects
which can in rare cases exceed the maximum number of failures.

The test was flaky because all invalid headers are counted as rejects
which can in rare cases exceed the maximum number of failures.

## Description
Change the test to assert that the validity status matches instead of
filtering those cases out

## Testing
- cargo test


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Co-authored-by: ysaito1001 <awsaito@amazon.com>