Fix archive filename matching, recent regression.

The census test didn't pass anymore because I broke the archive filename
matching in b17383fa90b81408fa08566bc131ea5b02606023, where the special
variable *csv-path-root* stoped being authoritative in the archive case.

To fix, initialize that variable to nil and give its value priority as
soon as it's non-nil, such as the archive case.
Add an hstore test

This test is currently commented out of the test suite so that we don't
require the hstore being available to run the basic tests.