Add test coverage for file entity reference selection plugin

Problem/Motivation

Creating a node with a entity reference field refer to a file got an error (on js console):
Fatal error: Call to a member function range() on a non-object in /core/modules/entity_reference/src/Plugin/entity_reference/selection/SelectionBase.php on line 184

Steps To Reproduce

Create a content type "File", with a file field "File"

Add some contents to "File"

Add reference field "File Reference" to "Article", then set "Type of item to reference" to "File" ("File" entity type, not "Content" entity type with bundle "File")

Open javascript console, then add new "Article"

Expected: autocomple work, with a list of file including sample.jpg
Actual: autocomple does not work, with an error: