client.cmdparser: prevent autotest-local from listing unavailable tests

The output of <autotest-local list> 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

so, we need a better criteria to list available tests for
existence of a control file. This patch fixes the problem.