fix(core_test): Increase docker timeout to 60 seconds

The first run of docker can take quite some time since docker isn't very
quick to start currently and fetching the busybox container can take
pretty much any length of time. Increasing to 60 seconds to be safe.
In a local KVM vm it currently takes 38 seconds for me.
Merge pull request #6 from marineam/docker-timeout

fix(core_test): Increase docker timeout to 60 seconds