selftest: wic: fix test_qemu

selftest: wic: fix test_qemu

Setting WKS_FILE variable in qemux86-64 made wic test to
use wrong wks file to produce an image and resulted in
test_qemu failure.

Used conditional assignment in qemux86-64 and explicitly
set WKS_FILE in wic testing suite to make the suite to use
wic-image-minimal.wsk. This should fix test_qemu failure.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>