Tests for behavior of export with & in HTML and XML-based components

These are tests to nail down the current behavior of export with & exists in an HTML component or a problem (XML) component.

See STUD-91

& is OK in an HTML component, and there is an import/export roundtrip test that uses the toy course. Therefore I added & to an existing HTML component in the toy course.

& is not OK in a problem (XML-based component). I added a test that export will fail with a way to click to the failing component. The Studio preview view of the component shows the error, so this failure should be clear to users.

@jzoldak Please review.