Sys path guard

[ejgroene]

When tests modify sys.path and forget to undo their changes, the code running next might behave unexpectedly. It would be nice to have a fixture that can be used to modify sys.path which restores the old path when done.

Also, this fixture might be nice as a guard, which always checks if tests do not screw up sys.path.

Same idea for import, although that might be more subtle.