Update test_xccdf_values_in_ds.sh to account for Bash variables

Bash variables whose names start with `$XCCDF_VALUE_` are valid in SCE
content. The `$XCCDF_VALUE_` variables are exported by `oscap` so that
the SCE check can use XCCDF Values in its code.

tests/test_xccdf_values_in_ds.sh

@@ -5,7 +5,7 @@ if grep -iq "dangling reference to" "$DS" ; then
     printf "Found dangling reference in %s, ensure correct usage of the xccdf_value Jinja macro!" "$DS" >&2
     exit 1
 fi
-if grep -iq "xccdf_value" "$DS" ; then
+if grep -iq "[^$]xccdf_value" "$DS" ; then
     printf "Found obtrusive xccdf_value in %s, ensure correct usage of the xccdf_value Jinja macro!" "$DS" >&2
     exit 1
 fi