tests: add a new test for testing changing or clearing topics test-topic.t is getting bigger and bigger. This patch introduces a new test which includes test for clearing and changing topics. Since the tests are moved to a new file, this patch, removes those tests from test-topic.t also.

obscache: do not check of argument count of wrapped function After talking with the original author of this check, it appears that it never worked properly, since _readmarkers was always wrapped with @util.nogc.