tests: tweak lock file handling and timers

- when server are killed by a test case, do not wait for the server lock
  file to go away. These tests are mostly about client timeouts and the
  server will hang until killed.
- when killing a server successfully, check for a remaining lock file,
  log its existence and remove it.
- lower the delay timings on SLOWDOWN by half
- add SLOWDOWNDATA server command to only slow down the FTP data bytes,
  not the control ones.
- lower some timeout values

Closes #14835

tests/processhelp.pm

@@ -327,6 +327,8 @@ sub killpid {
                 }
             }
             last if(not scalar(@signalled));
+            # give any zombies of us a chance to move on to the afterlife
+            pidwait(0, &WNOHANG);
             portable_sleep(0.05);
         }
     }