helper coverage: add message about the command taking a long time.

Also suppresses output from the fuzzer itself.

infra/helper.py

@@ -227,7 +227,10 @@ def coverage(run_args):
   ] + args.fuzzer_args
 
   print('Running:', _get_command_string(command))
-  pipe = subprocess.Popen(command)
+  print('This may take a while (running your fuzzer for %d seconds)...' %
+        args.run_time)
+  with open(os.devnull, 'w') as f:
+    pipe = subprocess.Popen(command, stdout=f, stderr=subprocess.STDOUT)
     pipe.communicate()
 
   command = [