Make context and test part of "expectation" class

[krlmlr]

so that add_result = function(result) { ... }, to support easier aggregation of results (#380 (comment)).

[hadley]

I like the idea of formalising these as fields in the expectation class (I think I'm informally tacking them on at the moment). But I do like the current symmetry of the reporter callbacks where as you get more and more detailed you get more and more arguments.

[krlmlr]

It's not terribly important anymore, and the reporter is a beautiful peace of ASCII art indeed:

  public = list(
    start_reporter = function() {},
    start_context =  function(context) {},
    start_test =     function(context, test) {},
    add_result =     function(context, test, result) {},
    end_test =       function(context, test) {},
    end_context =    function(context) {},
    end_reporter =   function() {}
  )