[SECP256K1] make test count iteration configurable by environment variable

Summary:
```
You can set the test iteration count by command-line parameter but it is
very hard to control the iteration count used by make check and even
harder to control the count when this is run through Bitcoin Core's
build system. Using an environment var solves both problems.
```

Backport of [[bitcoin-core/secp256k1#851 | secp256k1#851]]

Test Plan:
  SECP256K1_TEST_ITERS=16 ninja check-secp256k1
Should succeed but:
  SECP256K1_TEST_ITERS=0 ninja check-secp256k1
Should fail with error `An iteration count of 0 or less is not allowed.`

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9375

Author: apoelstra

src/secp256k1/src/tests.c

@@ -5635,6 +5635,15 @@ int main(int argc, char **argv) {
     /* find iteration count */
     if (argc > 1) {
         count = strtol(argv[1], NULL, 0);
+    } else {
+        const char* env = getenv("SECP256K1_TEST_ITERS");
+        if (env) {
+            count = strtol(env, NULL, 0);
+        }
+    }
+    if (count <= 0) {
+        fputs("An iteration count of 0 or less is not allowed.\n", stderr);
+        return EXIT_FAILURE;
     }
     printf("test count = %i\n", count);