t: lib-gpg: flush gpg agent on startup

t: lib-gpg: flush gpg agent on startup

When running gpg-relevant tests, a gpg-daemon is spawned for each
GNUPGHOME used. This daemon may stay running after the test and cache
file descriptors for the trash directories, even after the trash
directory is removed. This leads to ENOENT errors when attempting to
create files if tests are run multiple times.

Add a cleanup script to force flushing the gpg-agent for that GNUPGHOME
(if any) before setting up the GPG relevant-environment.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Santiago Torres <santiago@nyu.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>