Fail when test resolution method is overspecified

Users may register `Runnable`s as asynchronous in one of two ways:

- Via callback (by defining the body function to have an arity of one)
- Via promise (by returning a Promise object from the body function)

When both a callback function is specified *and* a Promise object is
returned, the `Runnable`'s resolution condition is ambiguous.
Practically speaking, users are most likely to make this mistake as they
transition between asynchronous styles.

Currently, Mocha silently prefers the callback amd ignores the Promise
object. Update the implementation of the `Runnable` class to fail
immediately when the test resolution method is over-specified in this
way.