message: [OMPT] Add synchronization to threads_nested.c testcase
  The testcase potentially fails when a thread is reused.
  The added synchronization makes sure this does not happen.
  Patch provided by Simon Convent
diff: #include <omp.h>
  int main() {
  +
  + int condition = 0;
  int x = 0;
  omp_set_nested(1);
  #pragma omp parallel num_threads(2)
  {
  #pragma omp parallel num_threads(2)
  {
  -#pragma omp atomic
  - x++;
  + OMPT_SIGNAL(condition);
  + OMPT_WAIT(condition, 4);
  }
  }
  