Restitute a successful stat call to this test case.

Since the first stat call is made for it to deliberately fail, and we
reference st.st_mode without st having valid data, st.st_mode can contain
garbage and cause chmod to fail with EINVAL. We rerun stat and ensure it
succeeded.

Also make use of tt_abort_perror, to properly convey failure reasons to
the user.

src/test/test_config.c

@@ -221,12 +221,17 @@ test_config_check_or_create_data_subdir(void *arg)
   // and is private to the user.
   test_assert(!check_or_create_data_subdir(subdir));
 
+  r = stat(subpath, &st);
+  if (r) {
+    tt_abort_perror("stat");
+  }
+
 #if !defined (_WIN32) || defined (WINCE)
   group_permission = st.st_mode | 0070;
   r = chmod(subpath, group_permission);
 
   if (r) {
-    test_fail_msg("Changing permissions for the subdirectory failed.");
+    tt_abort_perror("chmod");
   }
 
   // If the directory exists, but its mode is too permissive