Detect xxd(1) at test time, not at configure time.

Index: tests/test-fsck
--- tests/test-fsck.orig
+++ tests/test-fsck
@@ -35,7 +35,7 @@ fi
 testname=$(basename "$1" .fsck)
 
 
-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