* lisp/mail/footnote.el: Tweak markers convention

Instead of using markers that are sometimes before and sometimes after
the [...] and using `insert-before-markers` to make sure those that are
are before stay before, always place them before, and make them
"move after"so they stay with their [...] without the need for

(footnote--current-regexp): Add arg to match previous style.
Include the start/end "tags" in the regexp.  Adjust all callers.
(footnote--markers-alist): Change position of POINTERS.
(footnote--refresh-footnotes, footnote--renumber)
(footnote--make-hole, footnote-delete-footnote)
(footnote-back-to-message): Adjust accordingly, mostly by using
`looking-at` instead of `looking-back`.
(footnote--make-hole): Always return footnote nb to use.
(footnote-add-footnote): Simplify call accordingly.

* test/lisp/mail/footnote-tests.el: New file.
1 job for master in 90 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
failed #1618


Name Stage Failure
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/progmodes/xref-tests.log
ELC lisp/ps-print-tests.elc
GEN lisp/ps-print-tests.log
ELC lisp/register-tests.elc
GEN lisp/register-tests.log
ELC lisp/replace-tests.elc
GEN lisp/replace-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds