Discourse.Computed tests refactoring

A small clean-up. I've split the big fixture at the top of the file into several smaller ones and put them directly inside relevant tests. Because tests do not reuse properties from the fixture (each of the tests use different computed property) it does not introduce any duplication and having fixture set up inside the test improves readability in my opinion (you don't have to jump to the top of the file to see what is the format string or what fields property is based on etc. - this information is now very close to the assertions using it).