Move node/tests/src/Functional/AssertButtonsTrait.php into node/tests/src/Traits

Problem/Motivation

AssertButtonsTrait is under namespace \Drupal\Tests\node\Functional

Traits: The third class is traits. It is possible to write a trait which will be discovered and can be used by the various different test types. Each test suite will discover the trait if it is placed in the tests/src/Traits directory, and the trait will have a namespace of \Drupal\Tests\$extension\Traits.

-- PHPUnit file structure, namespace, and required metadata

Proposed resolution

Move core/modules/node/tests/src/Functional/AssertButtonsTrait.php into core/modules/node/tests/src/Traits