Test fixtures can now be used by consuming modules

This enable use of fixtures in Dancer::Plugin::Interchange6

- schema attribute in Test::Role::Database renamed to ic6s_schema
  in order to avoid conflicts with Dancer::Plugin::DBIC's schema
- change ::Role::SQLite to use Temp::File instead of :memory: db
- add clear_database method to ::Role::SQLite to help remove temp
  file cleanly on Windows systems
- add t/var directory for temp file usage