takeCallerFailures

Removes and returns the failures raised by callbacks this test supplied and contained by the composition, oldest first.

A callback that throws while a wrapper is writing to its widget does not stop the composition - one misbehaving listener cannot be allowed to leave a window unable to answer state - and a test whose callback threw fails on it once the test ends. A test that provokes such a failure on purpose takes it from here and asserts on it; what it takes no longer fails the test.

A failure arrives here once the pass that provoked it has been driven, so take it after the awaitIdle or awaitEventsDelivered that settles the write - taking it before returns nothing and leaves the failure to end the test.