[Form] Moved TypeTestCase to the Test namespace

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #7175
| License       | MIT
| Doc PR        | symfony/symfony-docs#2500

Todo:
 - [x] Make a Doc PR
 - [x] Update the changelog and UPGRADE-3.md files

As discussed in #7175, since this class is documented as a practise to test your form types, it's good to move it to the `Test` namespace.

I am not sure about the deprecation, I thought things deprecated in 2.3 are removed in 3.0. Also, I think we shouldn't trigger a E_USER_DEPRECATED message.
Please correct me if I'm wrong, it's my first PR for the core code.