It includes a fallback list of fourccs. Otherwise we might miss some
DRM_FORMAT definition.
This happens in SLES12:
```
../testsuite/gdk/dmabufformats.c: In function ‘test_dmabuf_formats_basic’:
../testsuite/gdk/dmabufformats.c:29:56: error: ‘DRM_FORMAT_ABGR16161616F’ undeclared (first use in this function); did you mean ‘DRM_FORMAT_ABGR2101010’?
29 | g_assert_true (gdk_dmabuf_formats_contains (formats, DRM_FORMAT_ABGR16161616F, DRM_FORMAT_MOD_LINEAR));
```
The dmabuf texture tests are failing, so we don't run them in
ci, but the format tests are perfectly fine, so split them off.
Add some tests for GdkDmabufFormatsBuilder and for the new
gdk_dmabuf_formats_equal(), too.
