imapd-validate is a beefed up version of our nntpd-validate test
which hammers the server with parallel connections over regular
IMAP, IMAPS, IMAP+STARTTLS; and COMPRESS=DEFLATE variants of
each of those. It uses $START_UID:$END_UID fetch ranges to
reduce requests and slurp many responses at once to saturate
"git cat-file --batch" processes.
mbsync(1) also uses pipelining extensively (but IMHO
unnecessarily), so it was able to shake out some bugs in
the async git code.
Finally, we remove xt/cmp-imapd-compress.t since it's
redundant now that we have PublicInbox::IMAPClient to work
around bugs in Mail::IMAPClient.