Move with_custom_state to infra & add tests

[galke7]

Description

This PR refactors the testing infrastructure by moving the with_custom_state decorator from the core spec context (tests/core/pyspec/eth2spec/test/context.py) to the infrastructure context (tests/infra/context.py).

Why this change is necessary:
The with_custom_state function is a testing utility, not a core specification rule. Keeping it in the core module risked creating circular dependencies as the test suite grows. Moving it to tests/infra ensures better modularity and allows other test modules to import it without conflict.

Additionally, this PR adds a new test file tests/infra/test_custom_state.py to verify that the decorator correctly injects custom state balances and parameters, ensuring the utility works as expected in isolation.

Checklist

• Update documentation (if applicable)
• Add tests for new functionality (if applicable)
• Run make lint to check formatting
• Run make test to check tests

Relations

Fixes #4669

[leolara]

We should more test cases, specially that test both parameters with different values. Perhaps it is better if the test function returns what you are trying to assert over.

[galke7]

Thanks for the guidance.
I've updated the PR to address your points:

• Refactored to Return Values: The test function now returns the state balance directly for assertion, removing the side-effect list.

• Increased Coverage: I implemented a table-driven test (matrix) that verifies three scenarios:
  Standard values (32 ETH threshold)
  Custom values (16 ETH threshold)
  Boundary check (Balance == Threshold)