mock-fs is currently incompatible with node 4.x, but a fix is in progress
https://github.com/tschaub/mock-fs/issues/59
Since we are currently not actively developing the affected broccoli plugins,
the risk of disabling these tests is low, especially in the light of
improvements we get from node 4.x.
Once we add support for addedPaths then this implementation will be significantly faster than the original.
In the meantime we benefit from having stable output directory which solves issues with certain files disappearing
during rebuild of a tree that contains flatten and mergeTree plugins.
Closes#2418
Igor Minar
Marc Laval
Martin Probst