test-pingpxe: fix identification of ping from busybox

Well, it was indeed working "by chance", as calling readlink on a non-link
returns empty, which instead of getting [ to return non-zero because
empty string is not "busybox" got it to return non-zero because the
comparison operator had no first operand.

Signed-off-by: Yann Dirson <yann.dirson@vates.tech>

Author: ydirson

tests/install/conftest.py

@@ -172,7 +172,7 @@ def remastered_iso(installer_iso, answerfile):
     test "$eth_mac" = "$br_mac"
 fi
 
-if [ $(readlink "/bin/ping") = busybox ]; then
+if [ "$(readlink /bin/ping)" = busybox ]; then
     # XS before 7.0
     PINGARGS=""
 else