Cleanup redundancy in tests

[levithomason]

There is a lot of redundancy in tests. Mostly, findFoo() calls serve both query and assertion purposes. These are equivalent:

let gridClassName = render(<Grid />).findClass('sd-grid').props.className;
gridClassName.should.contain('sd-grid');

render(<Grid />).findClass('sd-grid');

Update all tests to remove their redundant assertions.