misc/cgo/testcshared: wait up to 1 second in main2.c

Wait longer in case the system is heavily loaded.

Fixes #18324.

Change-Id: If9a6da1cf32d0321302d244ee24fb3f80e54489d
Reviewed-on: https://go-review.googlesource.com/34653Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/testcshared/main2.c

@@ -21,7 +21,7 @@ int main(void) {
 
   // The descriptor will be initialized in a thread, so we have to
   // give a chance to get opened.
-  for (i = 0; i < 100; i++) {
+  for (i = 0; i < 1000; i++) {
     n = read(fd, buf, sizeof buf);
     if (n >= 0)
       break;