test suite: gnupg setup: give each individual test its own AGENT.log

This was always the intent.  However, the structure here arranged to
put the actual value of GNUPGHOME into the import script.

Instead, move the GNUGPHOME setting into the setup fragment, which is
evaluated by the importer.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>