t/lib-gpg: fix gpgconf stderr redirect to /dev/null

t/lib-gpg: fix gpgconf stderr redirect to /dev/null

In 29ff1f8f74 (t: lib-gpg: flush gpg agent on startup, 2017-07-20), a
call to gpgconf was added to kill the gpg-agent.  The intention was to
ignore all output from the call, but the order of the redirection needs
to be switched to ensure that both stdout and stderr are redirected to
/dev/null.  Without this, gpgconf from gnupg-2.0 releases would output
'gpgconf: invalid option "--kill"' each time it was called.

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