tests: assert that capture info was received before trying screenshot

If the source is not supported, we won't receive the capture
information. So the capture info (size/format) will be zeroed, and we
fail while trying to create a buffer for the screenshot with size/format
zeroed.

With this patch we fail if we don't receive the capture info, what makes
the failure reason more explicit.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>