tests: make run-tests fail early if no channel is found

I hit a weird corner case in run-tests where a test that caused an
exception to be raised was breaking everything with an unbound
variable error a few lines down because channel was never getting set
in this for loop. By adding an `else` clause to this for loop, we can
explode right away if we can't find a channel and give the developer a
better chance at figuring out what's going on.

tests/run-tests.py

@@ -1767,6 +1767,8 b' class TestSuite(unittest.TestSuite):'
                 if not v:
                     channel = n
                     break
+            else:
+                raise ValueError('Could not find output channel')
             channels[channel] = "=" + test.name[5:].split(".")[0]
             try:
                 test(result)