tests: don't run test-update-atomic.t on chg I am unable to find a good way to make `showwrites` extension in it to work with chg. Also putting the use of showwrites inside `if no-chg` will defeat the purpose of test, so I just made the test no-op on chg.

tests: don't run couple of tests related to extensions loading with chg The couple of files test extensions loading and debugging that. It sets `devel.debug.extensions` to True to do that. Either we have to restart chg if this config is set or just don't run these tests on chg. I tried the first way by adding the config option to chgserver.py list of config subsections but that does not seem to work.