なお、MatchersとMustMatchersは同時には使えない模様。
ScalaTest should vs must
その違いとは。
http://www.scalatest.org/user_guide/using_matchers
機能的には、特に無い。
Trait MustMatchers is an alternative to Matchers that provides the exact same meaning, syntax, and behavior as Matchers, but uses the verb must instead of should. The two traits differ only in the English semantics of the verb: should is informal, making the code feel like conversation between the writer and the reader; must is more formal, making the code feel more like a written specification.