py.test has this curious feature 'xfail', that allows you to ensure a test execute, but expect it to fail. Not sure I've had a need for it.
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I love how the CommonMark Spec has a test suite that's just a JSON array. It's really easy to test a library for compliance, and I've seen developers nerd-sniped into full compliance.
https://spec.commonmark.org/0.31.2/spec.json
Assertions are a surprisingly nuanced design space. In a test, if I assert `x < y`, I really want to see the values of x and y when it fails.
Do you define an API for every possible predicate (Python's assertLess, expect.js) or try to support the native syntax (c.f. pytest)?
I'm still experimenting with UIs for live (sandboxed) evaluation of tests. I've realised that you really want to highlight the failing assertion, not just the failing test.
Feedback welcome :)