qi_add_test: Correctly reset variable before calling find_program.

Change-Id: Ifb5f8fedd481c092ed8d768f73f4dce7515a1774
Reviewed-on: http://gerrit.aldebaran.lan:8080/19066
Tested-by: gerrit
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Reviewed-by: mnottale <mnottale@aldebaran-robotics.com>
Tested-by: mnottale <mnottale@aldebaran-robotics.com>
cmake: backport qibuild3 changes

Change-Id: I84f17612b90404fbaaad0a5c2e7bef77f7a1bec3
Reviewed-on: http://gerrit.aldebaran.lan:8080/19099
Tested-by: gerrit
Reviewed-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>
Tested-by: dmerejkowsky <dmerejkowsky@aldebaran-robotics.com>