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;