Move helpers in file_test.module and delete it

Problem/Motivation

.module files are only necessary when there are preprocess functions now that almost all hooks can be OOP

Steps to reproduce

Open file

Proposed resolution

Move const to an enum:
FILE_URL_TEST_CDN_1
FILE_URL_TEST_CDN_2

Move file_test_get_return to FileTestHooks because it is only called by that one hook.

Move everything else to FileTestHelper

Delete file_test_validator it is not called anywhere.

Remaining tasks

None

User interface changes

N/A

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A