tests/docker: allow user to override check target

This is useful when trying to bisect a particular failing test behind
a docker run. For example:

  make docker-test-clang@fedora \
    TARGET_LIST=arm-softmmu \
    TEST_COMMAND="meson test qtest-arm/qos-test" \
    J=9 V=1

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221117172532.538149-4-alex.bennee@linaro.org>
2 files changed