test: fix test database backup/restore location

test: fix test database backup/restore location

backup_database() and restore_database() used to store the backups in
the test specific temporary directory, through the current working
directory being there. Commit 8e7fb88237ae ("test: use source and
build paths in test-lib-common.sh") started using a test specific
backup directories under the build tree test directory. This was in
error. Switch back to the old location, but using paths to the
location instead of relying on current working directory.

Reported by Daniel Kahn Gillmor <dkg@fifthhorseman.net>.