t9001: use test_when_finished

t9001: use test_when_finished

The confirmation tests in t9001 all save the value of
sendemail.confirm, do something to it, then restore it at
the end, in a way that breaks the &&-chain (they are not
wrong, because they save the $? value, but it fools
--chain-lint).

Instead, they can all use test_when_finished, and we can
even make the code simpler by factoring out the shared
lines.

Note that we can _almost_ use test_config here, except that:

  1. We do not restore the config with test_unconfig, but by
     setting it back to some prior value.

  2. We are not always setting a config variable. Sometimes
     the change to be undone is unsetting it entirely.

We could teach test_config to handle these cases, but it's
not worth the complexity for a single call-site.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>