Add TestCase assertion methods for testing checks

This patch adds assertCheckPasses and assertCheckFails methods to make
it easier to write tests for more complicated checks that we want to
add.

We rely on checks being generators at this point, meaning this will not
work on flake8 class based plugins.

Change-Id: I37ecec271006084211e37c4fc9c89d71df01dc97
Merge "Add TestCase assertion methods for testing checks"