[DocsUI, Search]: Fix and expand search tests.

Removes the test features that no longer are part of SearchLoader.
Splits test into parametrized and plain parts. Adds a test that
validates the cursor stack.

Bug: 391237353
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I39d20052d8f7e47924626a7adea5bab580601c01