Name the fields of the config test cases

The test cases of the configuration and the data directory set their
fields by name, with one field on each line, as the go-code skill
requires. A case then shows which value is the XDG directory and which
is the home directory without a look at the struct.

A search of the rest of the code finds no other positional struct
literal. The node keys stay positional, because the skill allows a small
two-field type that acts as one value.

Issue: b14d537