Improve broken protocol test generation (#3726)

We currently "hotfix" a broken protocol test in-memory, but there's no
mechanism that alerts us when the broken protocol test has been fixed
upstream when updating our Smithy version. This commit introduces such a
mechanism by generating both the original and the fixed test, with a
`#[should_panic]` attribute on the former, so that the test fails when
all its assertions succeed.

With this change, in general this approach of fixing tests in-memory
should now be used over adding the broken test to `expectFail` and
adding the fixed test to a `<protocol>-extras.smithy` Smithy model,
which is substantially more effort.

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