check(): Stricter check in tests

Previous version would give false 'OK' if, for example, we were supposed
to get [1, 2, 3] but instead got [2, 1, 3].
diff --git a/example/eigen.py b/example/eigen.py
index 57c516a..7ff9aed 100644
--- a/example/eigen.py
+++ b/example/eigen.py
@@ -22,7 +22,7 @@
 
 
 def check(mat):
-    return 'OK' if np.sum(mat - ref) == 0 else 'NOT OK'
+    return 'OK' if np.sum(abs(mat - ref)) == 0 else 'NOT OK'
 
 print("should_give_NOT_OK = %s" % check(ref[:, ::-1]))