Detect xxd(1) at test time, not at configure time.
Base cmp(1) doesn't support --bytes.

Index: tests/test-mkfs
--- tests/test-mkfs.orig
+++ tests/test-mkfs
@@ -39,7 +39,7 @@ fi
 testname=$(basename "$1" .mkfs)
 
 
-if [ "$XXD_FOUND" != "yes" ]; then
+if ! command -v xxd >/dev/null 2>&1; then
 	echo "xxd not available, required by test"
 	exit 77  # report test skipped
 fi
@@ -60,7 +60,7 @@ limitarg=
 if [ -n "$CMP_LIMIT" ]; then
 	limitarg="--bytes=$CMP_LIMIT"
 fi
-cmp $limitarg "${testname}.out" "${testname}.refimg"
+gcmp $limitarg "${testname}.out" "${testname}.refimg"
 success=$?
 
 if [ $success -eq 0 ]; then