Right now that attribute could be set by each test separately, or even varied within a test.

TBH I'm not sure that the attribute really should be supported; perhaps thinking about breaking the API is worth doing.

But - what are we solving for here. The OP here seems interested in using the assertion like things entirely outside of a test context.

What would a nice clean API for that be? (Yes I like matchers, but put that aside - if the APIs aren't close enough, lets make sure we do a good job for each audience rather than a compromise..)