tests: add test showing change of phase while changing topic on secret commit

topics: use scmutil.cleanupnodes to create markers while changing topics Earlier if we change topics on changesets with bookmark, the bookmark is not moved to the new changeset as we didn't add that logic and we were creating obsmarkers directly. This patch calls scmutil.cleanupnodes to create obsmarkes which also handles the bookmark movement.

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.