minor #17564 [Serializer] Use ::class in new tests (dunglas)
minor #17564 [Serializer] Use ::class in new tests (dunglas)

This PR was merged into the 3.1-dev branch.

Discussion
----------

[Serializer] Use ::class in new tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/pull/17446#discussion_r50965366
| License       | MIT
| Doc PR        | n/a

Commits
-------

48e8041 [Serializer] Use ::class in new tests

symfony/Serializer,symfony/serializer