Unit tests for consdiffmgr module

Initial tests. These just try adding a few consensuses, looking
them up, and making sure that consensus diffs are generated in a
more or less reasonable-looking way.  It's enough for 87% coverage,
but it leaves out a lot of functionality.

src/or/consdiffmgr.h

@@ -34,5 +34,10 @@ int consdiffmgr_cleanup(void);
 void consdiffmgr_configure(const consdiff_cfg_t *cfg);
 void consdiffmgr_free_all(void);
 
+#ifdef CONSDIFFMGR_PRIVATE
+STATIC consensus_cache_entry_t *cdm_cache_lookup_consensus(
+                          consensus_flavor_t flavor, time_t valid_after);
+#endif
+
 #endif