run-tests: "fix" race condition in race condition fix

Laurent's commit 3203dfe341f9 still suffers from a race: by the
time the "job" function tries to assign to channels[channel], that
list has been truncated to empty.  The result is that every job
thread raises an IndexError.

Earlier, I tried an approach of correctly locking channels, but
that caused run-tests to hang on KeyboardInterrupt sometimes.

This approach is strictly hackier, but seems to actually work
reliably.

tests/run-tests.py

@@ -1526,13 +1526,16 b' class TestSuite(unittest.TestSuite):'
             channels[channel] = "=" + test.name[5:].split(".")[0]
             try:
                 test(result)
-                channels[channel] = ''
                 done.put(None)
             except KeyboardInterrupt:
-                channels[channel] = ''
+                pass
             except: # re-raises
                 done.put(('!', test, 'run-test raised an error, see traceback'))
                 raise
+            try:
+                channels[channel] = ''
+            except IndexError:
+                pass
 
         def stat():
             count = 0