When changing to use "GraphicBitmap" property instead of
"ImageURL", the import wasn't adapted and there was no test which
would warn of such an situation. This also changes the difference
between writer and impress where impress used "Graphic" and not
"GraphicBitmap" property.

Also adds missing tests for both writer and impress

Change-Id: Ieed629d2d37f7806d63e729b6ef23cd848593071
Reviewed-on: https://gerrit.libreoffice.org/50140Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>