move tests for NamedScope and DefaultScope under test/cases/scoping/

The scoping/default.rb and scoping/named.rb got moved under scoping/ in
commit 2b22564c4efaa63d4bbc006762838c4025c1bdca,
but the tests never did.
rename named_scope_test.rb to a proper file name

The file name should be name_scoping_test.rb and the class should be
`NamedScopingTest` according to ActiveRecord::Scoping::Name
split relation_scoping_test.rb's default scoping tests into another file
Merge pull request #10003 from tkhr/refactor-scoping-test-files

Reorganize scoping test files