Calling test() multiple times in the same test file is not actually
valid, and can cause the following test to hang forever. So let's stop
doing that in the one test that did so, and also prevent the same
mistake happening again. :^)
Throwing an exception on subsequent test() calls means that we don't
hang, the test will fail with missing output, and we get a log message
explaining why.
This fixes the issue that occurred when, after clicking an inline
paintable page would always scroll to the top. The problem was that
`scroll_an_element_into_view()` relies on `get_bounding_client_rect()`
to produce the correct scroll position and for inline paintables we
were always returning zero rect before this change.