tests: add warning message when /dev/urandom fails

in case this code should ever be used as an example, a warning is a nice
way of helping ensure insecure keys are not generated

src/tests.c

@@ -4919,6 +4919,7 @@ int main(int argc, char **argv) {
     } else {
         FILE *frand = fopen("/dev/urandom", "r");
         if ((frand == NULL) || fread(&seed16, sizeof(seed16), 1, frand) != sizeof(seed16)) {
+            fprintf(stderr, "WARNING: could not read 16 bytes from /dev/urandom; falling back to insecure PRNG\n");
             uint64_t t = time(NULL) * (uint64_t)1337;
             seed16[0] ^= t;
             seed16[1] ^= t >> 8;