test: adding missing round-trip test for graphviz format

[Becheler]

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

Adds a round-trip check for graphiz format to guard against regressions.

Motivation

PR #566 will need to assert against possible regression when replacing xpressive by hand-rolled string predicate.

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 31576481977 vs develop run 31574016769 (f6b5d98208).

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

No header-inclusion-weight changes.

Transitive Boost modules: 68 → 68 (0)

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 31576481902 vs develop run 31574016777 (f6b5d98208).

Job	Baseline	After	Delta
macos (clang, 14)	681	703	+22
macos (clang, 17)	641	663	+22
macos (clang, 20)	641	663	+22
ubuntu (clang-19, 14)	681	703	+22
ubuntu (clang-19, 17)	641	663	+22
ubuntu (clang-19, 20)	641	663	+22
ubuntu (clang-19, 23)	641	663	+22
ubuntu (gcc-14, 14)	827	874	+47
ubuntu (gcc-14, 17)	949	1010	+61
ubuntu (gcc-14, 20)	949	1010	+61
ubuntu (gcc-14, 23)	949	1010	+61
windows_msvc_14_3 (msvc-14.3)	962	962	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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