run-tests: set the thread name to the test name for info on error
This does not happen when running normal. But when fiddling around with
the test infrastructure, this helps a lot.
Old traceback messge
Exception in thread Thread-7:
Traceback (most recent call last):
...
new traceback message
Exception in thread test-something.t:
Traceback (most recent call last):
...
[ original upstream message ]