[docsui] use TestFilesRule.createTestFiles in tests

Replace individual init helpers with a single closure-based helper.
This is more flexible and allows uniform initialisation evewywhere.

Bug: 438634983
Flag: EXEMPT change to test
Test: atest DocumentsUIGoogleTests
Change-Id: I1a28456f62f0b4d537176ae13be5a5439ae1c00b