normalize setters and getters

[boneskull]

The main classes (Runnable, Test, Context, Suite, Mocha, etc.) have these setter/getter functions, e.g. Runnable#timeout(). Not all of these, however, implement the "getter" part, where it's supposed to just return the value if it receives no arguments.

Furthermore, there are some methods that might check if the first argument is undefined; they need to check length of arguments instead, because undefined or void 0 is a valid argument.