Tests fail when fusefat is not installed

[oesteban]

This line causes the crash of Popen https://github.com/nipy/nipype/blob/master/nipype/testing/utils.py#L62 when running tests without fusefat installed.

[chrisgorgo]

We should probably detect fusefat and skip the test if not installed.

[effigies]

We detect a failure of fusefat, but not an absence. Pushing a candidate fix...