tests/docker: add docs FEATURE flag and use for test-misc

The test-misc docker test fails on a number of images which don't have
the prerequisites to build the docs. Use the FEATURES flag so we can
skip those tests.

As the sphinx test fails to detect whatever feature we need to get
hxtool to work we drop them from debian9 so the windows build doesn't
attempt to build the docs.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200414200631.12799-7-alex.bennee@linaro.org>