derivepassphrase bug test-filesystem-isolation

Bug details: Isolate tests properly from the filesystem

Class	bug	This is clearly an actual problem we want fixed.
Present-in	0.1.0 0.1.1 0.1.2
Fixed-in	a980a643275de28f7715241790f199f947f637f4 (0.1.3)

Tests for the derivepassphrase command-line interface interact with the user’s configuration, and will in general fail, unrelatedly, if said user configuration is broken.

While there is code to isolate the filesystem during tests, it is not yet consistently applied across all tests.

(Discovered while working on configuration-directory-must-exist.)