client.cmdparser: prevent autotest-local from ouputting unavailable test...

...s

when using 'autotest-local list' to output a list of available tests,
which including unavailable test. for example,
[root@localhost client]# ./autotest-local list
Autotest prepackaged tests (/root/osc/autotest/client/tests)
 [Control]                                          [Description]
 sleeptest                                          Sleeptest
 sleeptest/sleeptest_control_unittest.py            None