Refactor binary.rs tests into modular submodules under binary/tests

[kosiew]

Which issue does this PR close?

• Closes Restructure binary.rs Tests into Dedicated Modules #16767

Rationale for this change

This refactor improves the organization and maintainability of the type coercion tests in the datafusion/expr-common crate.
By grouping related tests into separate modules (arithmetic.rs, comparison.rs, dictionary.rs, null_coercion.rs), it makes the test suite easier to navigate, extend, and review.

What changes are included in this PR?

• Moved all existing tests from binary.rs into dedicated submodules under binary/tests/
  • arithmetic.rs — arithmetic coercion and numeric tests
  • comparison.rs — comparison, like, binary ops, map and list coercion tests
  • dictionary.rs — dictionary coercion tests
  • null_coercion.rs — logical ops and null coercion tests
• Extracted common test macros into tests/mod.rs
• Ensured no test coverage was dropped during the move

Are these changes tested?

✅ Yes — This PR only reorganizes existing, already tested code.
All tests continue to run under the new module structure.

Are there any user-facing changes?

❌ No — This is an internal refactor of test code only.
There are no changes to user-facing APIs or behavior.

[comphead]

Thanks @kosiew I think it makes sense to split the huge file up

[kosiew]

thanks @comphead for your review.