Add a failing test for a race in newlib.

tests/threads/newlib_stdio_test.c

+#include <pthread.h>
+#include <stdio.h>
+#include <unistd.h>
+
+void *thr0(void* arg) {
+  fopen("/tmp/z0", "w");
+  return 0;
+}
+
+void *thr1(void* arg) {
+  usleep(500000);
+  printf("success, %d\n", 1);
+  return 0;
+}
+
+int main(int argc, char *argv[]) {
+  pthread_t t0, t1;
+
+  pthread_create(&t0, NULL, thr0, NULL);
+  pthread_create(&t1, NULL, thr1, NULL);
+
+  pthread_join(t0, NULL);
+  pthread_join(t1, NULL);
+
+  return 0;
+}