tests: simplify random_fe_non_zero (remove loop limit and unneeded …

…normalize)

`random_fe_non_zero` contains a loop iteration limit that ensures that
we abort if `random_fe` ever yielded zero more than ten times in a row.
This construct was first introduced in PR bitcoin#19 (commit 09ca4f3) for
random non-square field elements and was later refactored into the
non-zero helper in PR bitcoin#25 (commit 6d6102f). The copy-over to the
exhaustive tests happened recently in PR bitcoin#1118 (commit 0f86420).

This case seems to be practically irrelevant and I'd argue for keeping
things simple and removing it; if there's really a worry that the test's
random generator is heavily biased towards certain values or value
ranges then there should consequently be checks at other places too
(e.g. directly in `random_fe` for 256-bit values that repeatedly
overflow, i.e. >= p).

Also, the _fe_normalize call is not needed and can be removed, as the
result of `random_fe` is already normalized.

src/tests.c

@@ -2967,16 +2967,9 @@ static void random_fe(secp256k1_fe *x) {
 }
 
 static void random_fe_non_zero(secp256k1_fe *nz) {
-    int tries = 10;
-    while (--tries >= 0) {
+    do {
         random_fe(nz);
-        secp256k1_fe_normalize(nz);
-        if (!secp256k1_fe_is_zero(nz)) {
-            break;
-        }
-    }
-    /* Infinitesimal probability of spurious failure here */
-    CHECK(tries >= 0);
+    } while (secp256k1_fe_is_zero(nz));
 }
 
 static void random_fe_non_square(secp256k1_fe *ns) {

src/tests_exhaustive.c

@@ -70,16 +70,9 @@ static void random_fe(secp256k1_fe *x) {
 }
 
 static void random_fe_non_zero(secp256k1_fe *nz) {
-    int tries = 10;
-    while (--tries >= 0) {
+    do {
         random_fe(nz);
-        secp256k1_fe_normalize(nz);
-        if (!secp256k1_fe_is_zero(nz)) {
-            break;
-        }
-    }
-    /* Infinitesimal probability of spurious failure here */
-    CHECK(tries >= 0);
+    } while (secp256k1_fe_is_zero(nz));
 }
 /** END stolen from tests.c */