tests: explicitly flush output streams Some tests fail while running with chg because they do not flush their output streams. chgserver will make sure ui.flush is called after dispatch, but not after {ui,repo}setup. For other non-ui streams, it should be explicitly flushed since the request handler will use os._exit. This patch adds explicit flushes in test-bundle2-format.t, test-extension.t and test-obsolete.t. It will fix most test cases of them when running with chg.