Adds failing specs for requiring files from css manifests

I need to put this work on hold for a little bit, but someone else
may be able to pick up where I've left off.

Problems to solve:
* It looks like vendor directories from the spree_core gem are not
  getting added to the tenant asset path. I suspect an issue with
  `Spree::SpreeLandlord::Assets::Bootstrap#run on lines 19-22
* Need to figure out a reliable way to verify that scss has been
  compiled correctly. The formatting does not seem to be preserved
  which will make those specs brittle. It may be useful to use a
  css parser to compare the content in a way that ignores formatting
  and comments.