Extract thread-const-array-join example from level-ip

tests/regression/03-practical/33-thread-const-array-join.c

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+// Thread pool constant array sequential joining.
+// Extracted from concrat/level-ip.
+#include <pthread.h>
+#include <goblint.h>
+
+int data = 0;
+pthread_mutex_t data_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+void *thread(void *arg) {
+  pthread_mutex_lock(&data_mutex);
+  data = __VERIFIER_nondet_int(); // NORACE
+  pthread_mutex_unlock(&data_mutex);
+  return NULL;
+}
+
+int main() {
+  int threads_total = 4;
+  pthread_t tids[4];
+
+  // create threads
+  for (int i = 0; i < threads_total; i++) {
+    pthread_create(&tids[i], NULL, &thread, NULL); // may fail but doesn't matter
+  }
+
+  // join threads
+  for (int i = 0; i < threads_total; i++) {
+    pthread_join(tids[i], NULL);
+  }
+
+  return data; // NORACE (all threads joined)
+}