chore: Migrate 1.4 mcgregor_subgraphs.cpp with range-based for

[NNhanptnk]

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Base on : #593
Refactor pre-C++11 macro with range-based for loop.
This refactor only affects : test/mcgregor_subgraphs_test.cpp

Motivation

Testing

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 35700604224 vs develop run 35656582944 (daa6f09fff).

Header-inclusion weights (graph files pulling each direct dependency in):

No header-inclusion-weight changes.

Transitive Boost modules: 47 → 47 (0)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 35700604094 vs develop run 31585046312 (daa6f09fff).

Job	Baseline	After	Delta
macos (clang, 14)	703	408	-295
macos (clang, 17)	663	407	-256
macos (clang, 20)	663	407	-256
ubuntu (clang-19, 14)	703	408	-295
ubuntu (clang-19, 17)	663	407	-256
ubuntu (clang-19, 20)	663	407	-256
ubuntu (clang-19, 23)	663	407	-256
ubuntu (gcc-14, 14)	874	373	-501
ubuntu (gcc-14, 17)	1010	369	-641
ubuntu (gcc-14, 20)	1010	369	-641
ubuntu (gcc-14, 23)	1010	369	-641
windows_msvc_14_3 (msvc-14.3)	962	961	-1