Abstract class that supports test registration in FlatSpec and fixture.FlatSpec.

For example, this class enables syntax such as the following pending test registration in FlatSpec and fixture.FlatSpec:

"A Stack (when empty)" should "be empty" is (pending)
                                         ^

For example, this class enables syntax such as the following tagged test registration in FlatSpec and fixture.FlatSpec:

"A Stack (when empty)" should "be empty" taggedAs(SlowTet) in { ... }
                                         ^

This class also indirectly enables syntax such as the following regular test registration in FlatSpec and fixture.FlatSpec:

"A Stack (when empty)" should "be empty" in { ... }
                                         ^

However, this class does not declare any methods named in, because the type passed to in differs in a FlatSpec and a fixture.FlatSpec. A fixture.FlatSpec needs two in methods, one that takes a no-arg test function and another that takes a one-arg test function (a test that takes a Fixture as its parameter). By constrast, a FlatSpec needs only one in method that takes a by-name parameter. As a result, FlatSpec and fixture.FlatSpec each provide an implicit conversion from ResultOfStringPassedToVerb to a type that provides the appropriate in methods.