Use tar -t instead of -l because centos 6

Centos 6 tar appears to only support unix style tar args. Use tar -t
when we inspect test builds.

Change-Id: I91bf321cf6700d59956779a60f613eddd2856024

tests/test_functions.bash

@@ -50,7 +50,7 @@ function run_element_test() {
             echo "No image $dest_dir/image.tar found!"
             exit 1
         else
-            if tar -l $dest_dir/image.tar | grep -q /tmp/dib-test-should-fail; then
+            if tar -t $dest_dir/image.tar | grep -q /tmp/dib-test-should-fail; then
                 echo "Error: Element: $element, test-element $test_element should have failed, but passed."
                 exit 1
             else