When running `cargo test -p bridge-hub-rococo-runtime --features
runtime-benchmarks`, two of the Snowbridge benchmark tests fails. The
reason is that when the runtime-benchmarks feature is enabled, the
`NoopMessageProcessor` message processor is used. The Snowbridge tests
rely on the outbound messages to be processed using the message queue,
so that we can check the expected nonce and block digest logs.

This PR changes the conditional compilation to only use
`NoopMessageProcessor` when compiling the executable to run benchmarks
against, not when running tests.

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Co-authored-by: claravanstaden <Cats 4 life!>