perf-test/tag: add maildir sync tests

Today someone asked me the (reasonable) question of how much
performance impact there is from synching tags to maildir flags. It
turns out it is noticeable, about a 50% overhead compared to
non-synched tags (according to these tests).  In practice I don't know
if it's a big problem for users, since I don't know what fraction of
tagging operations involve "special" tags.