MAINTAINERS: Fix subsystem name for "Build and test automation"

MAINTAINERS: Fix subsystem name for "Build and test automation"

The subsystem name for the "Build test automation" section is
"-------------------------", because an actual subsystem name
line is missing:

  $ ./scripts/get_maintainer.pl -f tests/docker/docker.py
  "Alex Bennée" <alex.bennee@linaro.org> (maintainer:-----------------...)
  Fam Zheng <famz@redhat.com> (maintainer:-----------------...)
  "Philippe Mathieu-Daudé" <f4bug@amsat.org> (reviewer:-----------------...)
  qemu-devel@nongnu.org (open list:-----------------...)

Fix the issue by inserting a subsystem name line where
get_maintainer.pl expects it.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170921170209.9101-1-ehabkost@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>