exclude some bmp files from test to enable plugin update

This MR should help resolve the chicken/egg problem with regard to gimp#12249.

It (temporarily) removes some of the bmp sample files from testing so that fixing the bmp plugin won't break CI.

All of the removed files are either perfectly valid or at least good enough to sensibly read, so removing the requirement for them to fail isn't wrong either way.

Once the bmp plugin is upgraded, the tests can be re-added, but with requirement to succeed.