Don't pass variant in params, it's ignored

We're setting variant above, in request object directly
Don't create addition vars, use options[] directly
Use concat instead of +
Add variants to Template class
Don't pass hash as keys to #find method
Disable LookupContext's cache when looking for template
Rename _setup_options to _options_for_digest
Set format in finder
Add mocked disable_cache for FixtureFinder