testsuite: Only free the main mutex when and if all the worker threads are done

changes/weasel-testuite-thread-fixes

@@ -0,0 +1,3 @@
+  o Minor bugfixes:
+    - Testsuite:  In the util/threads test no longer free the test_mutex
+      before all worker threads have finished.

src/or/test.c

@@ -2356,13 +2356,14 @@ test_util_threads(void)
     }
     tor_mutex_release(_thread_test_mutex);
   }
-  tor_mutex_free(_thread_test_mutex);
 
   tor_mutex_acquire(_thread_test_start1);
   tor_mutex_release(_thread_test_start1);
   tor_mutex_acquire(_thread_test_start2);
   tor_mutex_release(_thread_test_start2);
 
+  tor_mutex_free(_thread_test_mutex);
+
   if (timedout) {
     printf("\nTimed out: %d %d", t1_count, t2_count);
     test_assert(strmap_get(_thread_test_strmap, "thread 1"));