tests: introduce helper for non-zero random_fe_test results

There are several instances in the tests where random non-zero field
elements are generated by calling `random_fe_test` in a do/while-loop.
This commit deduplicates all these by introducing a
`random_fe_non_zero_test` helper. Note that some instances checked the
is-zero condition via `secp256k1_fe_normalizes_to_zero_var`, which is
unnecessary, as the result of `random_fe_test` is already normalized (so
strictly speaking, this is not a pure refactor).

src/tests.c

@@ -115,6 +115,12 @@ static void random_fe_test(secp256k1_fe *x) {
     } while(1);
 }
 
+static void random_fe_non_zero_test(secp256k1_fe *fe) {
+    do {
+        random_fe_test(fe);
+    } while(secp256k1_fe_is_zero(fe));
+}
+
 static void random_group_element_test(secp256k1_ge *ge) {
     secp256k1_fe fe;
     do {
@@ -129,12 +135,7 @@ static void random_group_element_test(secp256k1_ge *ge) {
 
 static void random_group_element_jacobian_test(secp256k1_gej *gej, const secp256k1_ge *ge) {
     secp256k1_fe z2, z3;
-    do {
-        random_fe_test(&gej->z);
-        if (!secp256k1_fe_is_zero(&gej->z)) {
-            break;
-        }
-    } while(1);
+    random_fe_non_zero_test(&gej->z);
     secp256k1_fe_sqr(&z2, &gej->z);
     secp256k1_fe_mul(&z3, &z2, &gej->z);
     secp256k1_fe_mul(&gej->x, &ge->x, &z2);
@@ -3810,18 +3811,14 @@ static void test_ge(void) {
     }
 
     /* Generate random zf, and zfi2 = 1/zf^2, zfi3 = 1/zf^3 */
-    do {
-        random_fe_test(&zf);
-    } while(secp256k1_fe_is_zero(&zf));
+    random_fe_non_zero_test(&zf);
     random_field_element_magnitude(&zf);
     secp256k1_fe_inv_var(&zfi3, &zf);
     secp256k1_fe_sqr(&zfi2, &zfi3);
     secp256k1_fe_mul(&zfi3, &zfi3, &zfi2);
 
     /* Generate random r */
-    do {
-        random_fe_test(&r);
-    } while(secp256k1_fe_is_zero(&r));
+    random_fe_non_zero_test(&r);
 
     for (i1 = 0; i1 < 1 + 4 * runs; i1++) {
         int i2;
@@ -4138,10 +4135,7 @@ static void run_gej(void) {
         CHECK(!secp256k1_gej_eq_var(&a, &b));
 
         b = a;
-        random_fe_test(&fe);
-        if (secp256k1_fe_is_zero(&fe)) {
-            continue;
-        }
+        random_fe_non_zero_test(&fe);
         secp256k1_gej_rescale(&a, &fe);
         CHECK(secp256k1_gej_eq_var(&a, &b));
     }
@@ -4580,9 +4574,7 @@ static void ecmult_const_mult_xonly(void) {
         random_scalar_order_test(&q);
         /* If i is odd, n=d*base.x for random non-zero d */
         if (i & 1) {
-            do {
-                random_fe_test(&d);
-            } while (secp256k1_fe_normalizes_to_zero_var(&d));
+            random_fe_non_zero_test(&d);
             secp256k1_fe_mul(&n, &base.x, &d);
         } else {
             n = base.x;
@@ -4611,9 +4603,7 @@ static void ecmult_const_mult_xonly(void) {
         } while (secp256k1_ge_x_on_curve_var(&x));
         /* If i is odd, n=d*x for random non-zero d. */
         if (i & 1) {
-            do {
-                random_fe_test(&d);
-            } while (secp256k1_fe_normalizes_to_zero_var(&d));
+            random_fe_non_zero_test(&d);
             secp256k1_fe_mul(&n, &x, &d);
         } else {
             n = x;