[DocsUI] add testSort_type_descending_with_directories

This adds a test with two halves. One doesn't pass (and is disabled). A
follow-up commit will fix it.

Bug: 437258483
Flag: EXEMPT test only
Test: atest DocumentsUIGoogleTests:com.android.documentsui.sorting.SortingCursorWrapperTest
Change-Id: I21fd07abf692b5c73b74b2e1ee376a465d82db28