[openmp] [test] Use omp_testsuite.h instead of directly including pthread.h

OpenMP tests that use pthread functions include this header instead.
On Unix systems, this header includes pthread.h, while it provides
minimal implementations of the used pthread functions for Windows.

Differential Revision: https://reviews.llvm.org/D137746

Author: mstorsjo

openmp/runtime/test/tasking/bug_nested_proxy_task.c

@@ -8,7 +8,7 @@
 
 #include <stdio.h>
 #include <omp.h>
-#include <pthread.h>
+#include "omp_testsuite.h"
 #include "omp_my_sleep.h"
 
 /*

openmp/runtime/test/tasking/bug_proxy_task_dep_waiting.c

@@ -8,7 +8,7 @@
 
 #include <stdio.h>
 #include <omp.h>
-#include <pthread.h>
+#include "omp_testsuite.h"
 #include "omp_my_sleep.h"
 
 /*