context should not exist on before/after tests.

[jamestalmage]

IMO, since context is not shared in before and after hooks.
It should be an error to access it in either scenario.

(cherry picked from commit f298d72 on #243)

[jamestalmage]

This becomes easier to solve with a helpful Error message with the attached metadata from #243 .

function checkContextAccess(test) {
    if (test.metadata.type === 'before' || test.metadata.type === 'after') {
      throw new Error('test.context access is not allowed in before/after hooks');
    }
}

Object.define(Test.prototype, 'context', {
  get: function () {
    checkContextAccess(this);
    return this._context;
  }, 
  set: function (newContext) {
    checkContextAccess(this);
    this._context = context;
  }
});