Extract common attributed string attributes for spec

[theextremeprogrammer]

Looks like some of the attributes used throughout these tests are sometimes repeated in different tests. Might be worth looking at extracting these out to the top of the file (or maybe a fixture? not sure if it is worth it though) to help reduce some clutter in the tests?

https://github.com/derekleerock/Succinct/blob/b93b9cedbb1139ad8b22feb5453261c3d0fd60a4/SuccinctTests/NSAttributedString/NSAttributedString%2BAttributesSpec.swift#L11