fix: johnsop-test.cpp was not testing anything

[Becheler]

The test was simply printing.

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

• modernized test (type aliases, constexpr...)
• use BOOST_TEST on expected output

Motivation

Flickering CI test coverage.

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 31396364669 vs develop run 31385980713 (b7b6b155e8).

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 31396363701 vs develop run 31385980794 (b7b6b155e8).

Job	Baseline	After	Delta
macos (clang, 14)	681	681	0
macos (clang, 17)	641	641	0
macos (clang, 20)	641	641	0
ubuntu (clang-19, 14)	681	681	0
ubuntu (clang-19, 17)	641	641	0
ubuntu (clang-19, 20)	641	641	0
ubuntu (clang-19, 23)	641	641	0
ubuntu (gcc-14, 14)	827	827	0
ubuntu (gcc-14, 17)	949	949	0
ubuntu (gcc-14, 20)	949	949	0
ubuntu (gcc-14, 23)	949	949	0
windows_msvc_14_3 (msvc-14.3)	962	962	0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.80%. Comparing base (bc05361) to head (b7b6b15).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #556      +/-   ##
===========================================
+ Coverage    88.22%   92.80%   +4.58%     
===========================================
  Files          392      392              
  Lines        27412    28167     +755     
  Branches      7997     7996       -1     
===========================================
+ Hits         24183    26140    +1957     
- Misses        1881     1922      +41     
+ Partials      1348      105    -1243     

Files with missing lines	Coverage Δ
test/johnson-test.cpp	100.00% <100.00%> (ø)

... and 288 files with indirect coverage changes

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