Update zsys.c

The czmq self test "forgets" to delete the folder ".testsys" (correct me if I'm wrong)
On my Linux machine, this adjustment fixes the problem.

///////////////////////////////////////////

This pull request is wrongfully being "failed" by the testing system:
zsys: OK 
...
line 2185: lt-czmq_selftest: zbeacon.c:254: zbeacon_test: Assertion `*zbeacon_hostname (node3)' failed. 
-> this failure is the legacy from the previous merge