load each data set only the first time it's used (Fixes #330)

also, do not include data set load time in the timing for each test
keep temp files separate and clean up only the correct ones (Fixes #305)

The regression spec and the filesystem spec were each deleting the
entire temp directory, which meant they could delete each other's files
depending on timing.

This should fix the intermittent failures we've been seeing.
Merge pull request #333 from mossprescott/ready/330

Integration test fixes