test-iot: fix an overwrite issue of get_tests_list
test-iot: fix an overwrite issue of get_tests_list

Because test-iot had overwritten get_tests_list, when we do testimage,
testimage_main will call a wrong function, So change to: only
overwrite this function when do test_iot

Signed-off-by: zjh <2b052533cdbcec2c486b017cd9ef932a2d35e8d0@intel.com>

wanghongjuan/meta-iotqa-1,ostroproject/meta-iotqa,ostroproject/meta-iotqa,daweiwu/meta-iotqa-1,daweiwu/meta-iotqa-1,daweiwu/meta-iotqa-1,daweiwu/meta-iotqa-1,wanghongjuan/meta-iotqa-1,wanghongjuan/meta-iotqa-1,wanghongjuan/meta-iotqa-1,ostroproject/meta-iotqa,daweiwu/meta-iotqa-1,ostroproject/meta-iotqa,ostroproject/meta-iotqa,wanghongjuan/meta-iotqa-1