t/lib-gpg: kill all gpg components, not just gpg-agent

t/lib-gpg: kill all gpg components, not just gpg-agent

The gpg-agent is one of several processes that newer releases of GnuPG
start automatically.  Issue a kill to each of them to ensure they do not
affect separate tests.  (Yes, the separate GNUPGHOME should do that
already. If we find that is case, we could drop the --kill entirely.)

In terms of compatibility, the 'all' keyword was added to the --kill &
--reload options in GnuPG 2.1.18.  Debian and RHEL are often used as
indicators of how a change might affect older systems we often try to
support.

    - Debian Strech (old old stable), which has limited security support
      until June 2022, has GnuPG 2.1.18 (or 2.2.x in backports).

    - CentOS/RHEL 7, which is supported until June 2024, has GnuPG
      2.0.22, which lacks the --kill option, so the change won't have
      any impact.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>