Description

Just discussed in a lab.

To better facilitate integrate with third party assertion tools, there should be a dedicated Issue.Kind case for them. Because the current expectationFailed takes in a type that is still being iterated. This new case should take in a new, kind of lightweight version of Expectation so that we don't have to open up Expectation publicly.

One terrible name suggestion: thirdPartyExpectationFailed. Let's not use that, though.

Related to #474 & #481

Expected behavior

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Actual behavior

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Steps to reproduce

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swift-testing version/commit hash

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Swift & OS version (output of swift --version && uname -a)

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