Rename test-abi to test-symbols, start verifying symbols are documented

We keep forgetting to update `apidoc/ostree-sections.txt`, so let's
start enforcing it.  Of course it turns out we had some bugs here
like symbols marked as public but never implemented, etc.  Those
are fixed in the prior commits.

Closes: #263
Approved by: giuseppe

src/libostree/libostree.sym

@@ -323,11 +323,6 @@ global:
         ostree_deployment_unlocked_state_to_string;
 } LIBOSTREE_2016.3;
 
-/*                         NOTE NOTE NOTE
- * Versions above here are released.  Only add symbols below this line.
- *                         NOTE NOTE NOTE
- */
-
 LIBOSTREE_2016.5 {
 global:
         ostree_repo_import_object_from_with_trust;
@@ -337,3 +332,8 @@ global:
         ostree_repo_get_remote_boolean_option;
         ostree_repo_set_cache_dir;
 } LIBOSTREE_2016.4;
+
+/*                         NOTE NOTE NOTE
+ * Versions above here are released.  Only add symbols below this line.
+ *                         NOTE NOTE NOTE
+ */