Add missing test coverage for Batch API processing behaviour

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Problem/Motivation

Follow-ups of #2875151: [META] Implement Batch API as a service move form.inc and batch.inc code into a service. Reviewing that move as behaviour-neutral needs tests that pass before and after. These behaviours have no test:

• hook_batch_alter() is invoked and its changes are applied.
• The destination query argument is removed at batch start and restored at the end.
• Redirect precedence: finished callback response, form state redirect, batch_process() $redirect argument, source URL.
• A non-progressive batch run outside the Form API.
• The session batches key is removed after the last batch finishes.

Steps to reproduce

Not applicable.

Proposed resolution

Add tests and test fixtures only. The tests call batch_set(), batch_process() and batch_get(), so the refactor must pass them unchanged.

• batch_test: a state-gated hook_batch_alter() implementation, a recording callback, three test routes.
• ProcessingTest: testBatchAlter(), testDestinationPreserved(), testRedirectPrecedence(), testSessionCleanup().
• BatchKernelTest: testNonProgressiveOutsideForm().

Each test fails when the code it covers is temporarily broken.

Remaining tasks

• Review.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.