Extend coverage for Section 25 - Generators (#700)

* Improve tests for GeneratorPrototype methods

- Assert return values more consistently
- Expand tests for constraints on `this` value

* Add more tests for dynamic GeneratorFunctions

* Add more tests for GenerationFunction.prototype

* Add more tests for the GeneratorFunction object

* Extend test: GeneratorFunction.prototype.prototype

* Improve precision of tests for generator methods

Extend existing assertions to explicitly verify that execution halts at
the intended location. Correct tests which were previously asserting
this behavior in contexts that did not match their name/description.

* Remove unused variables

* fixup! Improve tests for GeneratorPrototype methods

* fixup! Improve tests for GeneratorPrototype methods