Merge bitcoin#851: make test count iteration configurable by environment variable

f4fa8d2 forbid a test iteration of 0 or less (Andrew Poelstra)
0ce4554 make test count iteration configurable by environment variable (Andrew Poelstra)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK f4fa8d2
  real-or-random:
    ACK f4fa8d2

Tree-SHA512: 087771402c8e9536c07446baa7d02da5104d2b691f40c1dd04737329534422d895d3b692f485990d5791af8ccc124305b4f8b19be75e27b6b04cfb2337b28beb

Author: jonasnick

src/tests.c

@@ -5626,6 +5626,15 @@ 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);