Fixes#3201.
Assertions now throw a `TestFailure` error when they fail. This error is not exported or documented and should not be used or thrown manually. You cannot catch this error in order to recover from a failure, use `t.try()` instead.
All assertions except for `throws` and `throwsAsync` now return `true` when they pass. This is useful for some of the assertions in TypeScript where they can be used as a type guard.
Committing a failed `t.try()` result now also throws.
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