Better inversion in tests (#30)

curve-tests/src/fields.rs

@@ -79,10 +79,10 @@ fn random_inversion_tests(rng: &mut R) {
 
     for _ in 0..ITERATIONS {
         let mut a = F::rand(rng);
-        let b = a.inverse().unwrap(); // probablistically nonzero
-        a *= &b;
-
-        assert_eq!(a, F::one());
+        let b = a.inverse().map(|b| {
+            a *= &b;
+            assert_eq!(a, F::one());
+        });
     }
 }