An integration test for `docker buildx version` has been created. The
integration test checks that there is one line output, the output is
composed of three sections, and that these sections could feasibly be
the package path, version, and revision information.
The intention of the checks is to find obvious errors in the output like
the package path not existing or the version and revision being swapped.
It is not intended to assert that these values must be certain values
because it is assumed these values may vary depending on the build
process for buildx.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>