testsuite: run_timeout.py: print an error on timeout

Problem: Sometimes it is difficult to determine that a test run
under t/scripts/run_timeout.py timed out when debugging test failures.
The script kills the process when it times out, but otherwise doesn't
log an error, and therefore this appears as silent failure.

Add a message to run_timeout.py when a process times out so there is
a hint in the test logs.

t/scripts/run_timeout.py

@@ -71,6 +71,7 @@ def do_timeout():
         r = p.wait(timeout=args.timeout)
     except s.TimeoutExpired:
         # send signal to timeout process
+        print(f"{args.cmd} timed out after {args.timeout}s", file=sys.stderr)
         p.send_signal(args.signal)
         if args.kill_after > 0:
             try: