Refactor class_test to use reflective_test_loader

[srawlins]

And move some common code to template_test_base.dart, and refer to shared code in class_test and extension_type_test.

Also, all of the existing test cases in class_test were about the "implementers" section, but I hadn't included that in the name, so I did that. Then I added some tests for constructors and instance methods showing up on a class's page. This is the groundwork for showing that macro-generated constructors and instance methods show up.

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[kallentu]

Nice. This is great :)