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/test/test.c

@@ -1197,6 +1197,7 @@ struct testgroup_t testgroups[] = {
   { "connection/", connection_tests },
   { "conscache/", conscache_tests },
   { "consdiff/", consdiff_tests },
+  { "consdiffmgr/", consdiffmgr_tests },
   { "container/", container_tests },
   { "control/", controller_tests },
   { "control/event/", controller_event_tests },