docker: cleanup temp directory after test

docker: cleanup temp directory after test

There are temp directories named "docker-src.*" after doing docker
tests.  I don't see much point in keeping that (it only contains the
qemu.tar which is exactly current tree, and the copied "run" file).
Let's remove it after test finished.

CC: "Alex Bennée" <[email protected]>
CC: Fam Zheng <[email protected]>
CC: "Philippe Mathieu-Daudé" <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
Message-Id: <20171017071247[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>