ResourcesTest: Check if Docker supports limits

Check if Docker supports the limits we are trying to set (memory, swap),
and modify our expectations based on that.

This requires docker-client 2.6.5, which includes Info.memoryLimit() and
Info.swapLimit() for checking whether the limits are supported.

Also includes a tweak to use CPUSET="0", which is the only setting for
cpuset which works on CircleCI.