Common tests

[mrginglymus]

Adding a new test was a bit of a PITA due to the sheer number of libraries involved.

This proposed change (with example implementations in riot and react) moves all test definitions to a common location, with frameworks only having to invoke them with a set of factory functions.

This should hopefully make it easier to:

1. Add a new test across all libraries (libraries missing implementations will gracefully skip)
2. Add a new library (tests missing implementations will gracefully skip)
3. Critically, ensure consistency of behaviour in tests

I can't see what skips will look like until #2475 is merged :)

I may pick off a few others to test this works, but I wont commit to completing until it's approved in principle.