Fix broken MRAP tests

[isaiahvita]

1. Fix test case custom endpoint with hostname immutable. The problem was that for custom v1 resolvers with immutable hostname. we were previously disallowing arn buckets. but MRAP buckets are ARNs, so i modified the serialize immutable hostname middleware to allow MRAP buckets (which are identified as region-less).
2. Fixed several tests which were previously succeeding but erroring out on EP20. Confirmed in SEP that these tests shouldnt be succeeding and that EP20's behavior is more correctly following the SEP.
3. For v4a, the EP20 middleware was setting the signer name as sigv4a when the signer middleware was switchcasing off of SigV4A. so i modified codegen to set SigV4A instead
4. Fixed a couple of mismatched error messages
5. Fixed some broken unit tests for same issue as convert s3 handle_200_error_test into an actual unit test #2174