tests.linux-tools: bugfixes

linux-tools tests suffer from the following problem,
1): crontab: test does not exist
2): pax    : test does not exist

for example,
[root@localhost client]# ./autotest-local run linux-tools/crontab
10:52:56 INFO | Writing results to /root/osc/autotest/client/results/default
10:52:56 INFO | START   ----    ----    timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 INFO |     START   crontab.normal_cron crontab.normal_cron timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 ERROR| child process failed
10:52:56 INFO |         ERROR   crontab.normal_cron crontab.normal_cron timestamp=1416538376    localtime=Nov 21 10:52:56   crontab: test does not exist
10:52:56 INFO |     END ERROR   crontab.normal_cron crontab.normal_cron timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 INFO |     START   crontab.deny_cron   crontab.deny_cron   timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 ERROR| child process failed
10:52:56 INFO |         ERROR   crontab.deny_cron   crontab.deny_cron   timestamp=1416538376    localtime=Nov 21 10:52:56   crontab: test does not exist
10:52:56 INFO |     END ERROR   crontab.deny_cron   crontab.deny_cron   timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 INFO |     START   crontab.allow_cron  crontab.allow_cron  timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 ERROR| child process failed
10:52:56 INFO |         ERROR   crontab.allow_cron  crontab.allow_cron  timestamp=1416538376    localtime=Nov 21 10:52:56   crontab: test does not exist
10:52:56 INFO |     END ERROR   crontab.allow_cron  crontab.allow_cron  timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 INFO | END GOOD    ----    ----    timestamp=1416538376    localtime=Nov 21 10:52:56
10:52:56 INFO | Report successfully generated at /root/osc/autotest/client/results/default/job_report.html