mandos-ctl: Refactor

* mandos-ctl (milliseconds_to_string): Move to
  "PrintTableCmd.TableOfClients.milliseconds_to_string".
  (PrintTableCmd.TableOfClients.valuetostring): Change to a class
                                                method to be able to
                                                call
                                         cls.milliseconds_to_string().
  (TestCmd.other_client): Change "ApprovalDuration" to a more odd
                          value to compensate for the removal of
                          "Test_milliseconds_to_string", which
                          contained a test for this value.  All users
                          changed.
  (Test_milliseconds_to_string): Remove.