Fix auth failures in codegen-client-tests in orchestrator mode (#2812)

## Motivation and Context
In orchestrator mode, most `codegen-client-test` tests were failing due
to being unable to find a matching auth scheme, or due to some of the
test models referencing the `@sigv4` trait. This PR fixes all of those
failures, and adds the `smithy.runtime.mode` flag to
`codegen-client-test` as well.

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