Add a CLI test for multipage export with filters

Create a CLI test checking if objects with rasterized filter effects are
correctly exported to multipage PDFs when placed on pages other than the
first one. Add support for extracting a single page from a multipage PDF
to the existing helper scripts for raster output comparison in CLI tests.
Change the default shell for custom check scripts from "sh" to "bash".

Reference: inkscape/inkscape#3214