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):
...

tests/run-tests.py

@@ -1096,7 +1096,7 b' def scheduletests(options, tests):'
                 test = tests.pop(0)
                 if options.loop:
                     tests.append(test)
-                t = threading.Thread(target=job, args=(test, count))
+                t = threading.Thread(target=job, name=test, args=(test, count))
                 t.start()
                 running += 1
                 count += 1