test/ipfs-test-lib: add docker support

We have to do something special for CircleCI in docker_exec()
because "docker exec" doesn't work on CircleCi:

https://circleci.com/docs/docker#docker-exec

We indeed get "Unsupported: Exec is not supported by the lxc
driver" with CircleCi, when using "docker exec".

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

circle.yml

@@ -3,6 +3,7 @@ machine:
     TEST_NO_FUSE: 1
     TEST_VERBOSE: 1
     TRAVIS: 1
+    CIRCLE: 1
   post:
     - sudo rm -rf /usr/local/go
     - if [ ! -e go1.5.linux-amd64.tar.gz ]; then curl -o go1.5.linux-amd64.tar.gz https://storage.googleapis.com/golang/go1.5.linux-amd64.tar.gz; fi