Issue #1928752 by mtift, jenlampton: Put all test modules into the 'testing'...

Issue #1928752 by mtift, jenlampton: Put all test modules into the 'testing' Fieldset on the modules page (file module).

core/modules/file/tests/file_module_test/file_module_test.info.yml

 name: 'File test'
 type: module
 description: 'Provides hooks for testing File module functionality.'
-package: Core
+package: Testing
 version: VERSION
 core: 8.x
 hidden: true