Fix relative residual formula in tests

Author: ekatralis

tests/test_sparse_solvers.py

@@ -12,7 +12,7 @@
 The relative residual can be defined as:
                                || A * x - b ||
                         η = ---------------------
-                             ||A||*||x|| + ||b||
+                                    ||b||
 
 Typically, the expected value for this quantity is:
 * Ideally: 1e-12 - 1e-14

xobjects/sparse/_test_helpers.py

@@ -21,7 +21,7 @@ def rel_residual(A,x,b):
         b = b.get()
     assert issparse(A), ("A must be a sparse matrix")
 
-    return npl.norm(A@x - b) / (scspl.norm(A)*npl.norm(x) + npl.norm(b))
+    return npl.norm(A@x - b) / (npl.norm(b))
 
 def assert_residual_ok(res_ref, res_solver,
                        abs_tol=1e-12,