No global state for isolated tests and other fixes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

By default, phpunit preserves global state for isolated processes. This made the tests break on my laptop.
Other tweaks included.
In branch 2.5, `src/Symfony/Component/Security/Csrf/Tests/TokenStorage/NativeSessionTokenStorageTest.php` also misses the `@preserveGlobalState disabled` annotation. Please add it when merging