Add message to test output if a test is aborted due to a timeout.

Without this an aborted test could produce an empty .err file so the diff
looks like as if all lines of the .out files are just missing for some unknown
reason.

tests/run-tests.py

@@ -227,6 +227,8 b' def run(cmd):'
             ret = proc.wait()
             if ret == 0:
                 ret = signal.SIGTERM << 8
+            output += ("\n### Abort: timeout after %d seconds.\n"
+                       % options.timeout)
     return ret, splitnewlines(output)
 
 def run_one(test):