docker: add test-unit runner

This test doesn't even build QEMU, it just builds and runs all the
unit tests. Intended to make checking unit tests on all docker images
easier.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

tests/docker/test-unit

+#!/bin/bash -e
+#
+# Build and run the unit tests
+#
+# Copyright (c) 2018 Linaro Ltd.
+#
+# Authors:
+#  Alex Bennée <alex.bennee@linaro.org>
+#
+# This work is licensed under the terms of the GNU GPL, version 2
+# or (at your option) any later version. See the COPYING file in
+# the top-level directory.
+
+. common.rc
+
+cd "$BUILD_DIR"
+
+# although we are not building QEMU itself we still need a configured
+# build for the unit tests to be built and run
+configure_qemu
+check_qemu check-unit