tests: adds r = s = -n ecdsa verify test case

test/fixtures/ecdsa.json

@@ -207,6 +207,15 @@
           "r": "02",
           "s": "fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141"
         }
+      },
+      {
+        "description": "Invalid r, s values (r = s = -n)",
+        "d": "01",
+        "e": "01",
+        "signature": {
+          "r": "-115792089237316195423570985008687907852837564279074904382605163141518161494337",
+          "s": "-115792089237316195423570985008687907852837564279074904382605163141518161494337"
+        }
       }
     ]
   }