Log assertion failures right after test result

Log assertion failures right after test result

The extra messages are typically traceback from assertion failures.
Previously, they'd be printed only after all individual test case
results have been printed. That made it difficult to pair the traceback
to the failing test in some cases, as the node information (aka test
name) might not always be present.

Instead, log any extra messages related to a particular test failure
directly after reporting its result, making the failure details more
readily available and easy to connect with a particular test case.