…malizer context (dbu)

This PR was merged into the 4.2 branch.

Discussion
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[Serializer] Also validate callbacks when given in the normalizer context

| Q             | A
| ------------- | ---
| Branch?       | 4.2 (callbacks are handled differently in 3.4)
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no (unless somebody relied on this bug ignoring `null` as callback
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | Related to #30888
| License       | MIT
| Doc PR        | -

callbacks configuration for the normalizer is validated to be valid callbacks when using setCallbacks or using the callbacks field in the default options. however, it was not validated when using the callbacks field in a context passed to `normalize()`

Commits
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3789152 [serializer] validate that the specified callbacks and max_depth_handler are actually callable

This PR was squashed before being merged into the 4.3-dev branch (closes #30888).

Discussion
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[serializer] extract normalizer tests to traits

eufossa

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | Relates to #30818
| License       | MIT
| Doc PR        | -

As discussed with @joelwurtz, extract normalizer functionality tests into traits to ensure consistent behaviour of all normalizers.

* [x] Rebase when #30977, #30950 and #30907 are merged to master **blocker**
* [x] Clean up order of trait inclusion and methods in the tests
* [x] Clean up fixture classes of the traits. I started having one class named the same as the trait, where possible

Stuff that we should do eventually, but can also do in separate pull requests, after this one has been merged:
* [ ] Extract all features that we can (the existing normalizer tests should more or less only have the legacy tests in them, all functionality should be in trait)
* [ ] Run test coverage and increase coverage so that we cover all important features and all relevant error cases.

Commits
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2b6ebea [serializer] extract normalizer tests to traits