Add test for Klever's multiple threads

tests/regression/51-threadjoins/07-klever-multiple.c

@@ -0,0 +1,24 @@
+//PARAM: --set ana.activated[+] threadJoins --set lib.activated[+] klever
+#include <pthread.h>
+#include <goblint.h>
+
+int g = 0;
+
+void *t_fun(void *arg) {
+  g++; // RACE!
+  return NULL;
+}
+
+int main() {
+  pthread_t id;
+  pthread_create_N(&id, NULL, t_fun, NULL); // spawns multiple threads
+  pthread_join(id, NULL);
+
+  g++; // RACE!
+
+  pthread_join_N(id, NULL); // TODO: should this join one (do nothing) or all (like assume join)?
+
+  g++; // RACE!
+
+  return 0;
+}