Honestly: I threw Java 8 into the title when I first started writing, to tell myself that the book would seek to take advantage of the newer features to help explain unit testing. When finished, I realized that Java 8 or not was a non-issue--it makes for tighter code (often, not always), but it wasn't a primary focus of the book as I'd originally intended. That's ok--the important stuff is the concepts and approaches to unit testing.
For someone who doesn't know Java 8, my thought was that it's fairly easily read and understood. Reading a short article should be enough to clarify the use of lambdas in the book for anyone not familiar.
My personal bent is to prod advances in the language, and maybe push people a little. I hope for the book to survive at least 5 more years, and as such I preferred using 8 (which is almost a year into GA at this point). I also prefer a more functional approach given that I've been doing a bunch of Clojure lately.
As far as the title: I realized late in the game that I'd prefer not mentioning the Java version, but PragProg prefers it that way for now--I think we de-emphasized it a bit.