xt/eml_check_limits: check limits against an inbox

xt/eml_check_limits: check limits against an inbox

This allows maintainers to easily check limits against the
contents of existing inboxes.  This script covers most of
the new limits enforced by PublicInbox::Eml.

Usage is similar to most xt/*.t scripts:

  GIANT_INBOX_DIR=/path/to/inbox prove -bvw xt/eml_check_limits.t

Setting `TEST_CLASS=PublicInbox::MIME' allows us to check
performance and memory use against the old subclass of
Email::MIME.