test-hgrc: do not print ui.plain() flag at uisetup()
This makes the test pass with chg. Since uisetup() is run per process,
"hg showconfig" does not always call uisetup().
tests: make config/help tests pass even if chgserver extension is loaded
It isn't important for these tests if "extensions.chgserver=" is set or not.
So let's make them ignore such lines.