mandos-ctl: Refactor: fix confusion about values_to_set/values_to_get

* mandos-ctl (TestPropertySetterCmd.runTest): Generalize to work on
                                              any command regardless
                                              of what combination of
                                              values_to_set or
                                              values_to_get attributes
                                              it has.
  (TestPropertySetterCmd.run_command): Remove.
  (TestEnableCmd, TestDisableCmd, TestBumpTimeoutCmd,
  TestStartCheckerCmd, TestStopCheckerCmd, TestApproveByDefaultCmd,
  TestDenyByDefaultCmd): Change "values_to_set" to "values_to_get",
                         since that is what is intended.
  (TestPropertySetterValueCmd): Remove; all inheritors changed to
                                inherit from TestPropertySetterCmd.