test/v2: Resolve cmake link error regarding init_unit_test_suite

[nigels-com]

Fix for boostorg/boost Issue #1033

I'd come across a problem building boost 1.89.0 on Ubuntu 24.04 (Noble) in -DBUILD_TESTING=Y mode.
Turned out the same had been reported previously as mentioned above.

This change resolves the problem for me, seems like the same as being done for test/v1 tests.

Working build for boostorg/boost

cmake .. --fresh -G Ninja -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTING=Y && ninja

[nigels-com]

MS copilot says:

The pull request "test/v2: Resolve cmake link error regarding init_unit_test_suite" addresses build issues experienced when building Boost 1.89.0 on Ubuntu 24.04 with testing enabled, specifically related to the init_unit_test_suite link error noted in boostorg/boost Issue #1033. The PR updates seven test files in the test/v2 directory to add #define BOOST_TEST_MAIN at the top of each file. This aligns the v2 tests with the solution already used in the v1 tests.

• No files were deleted and only straightforward additions were made (no deletions).
• The change is minimal and targeted, reducing risk.
• The PR is marked as mergeable, but with an "unstable" mergeable state (which may be due to CI or branch status).
• There are no code comments, requested reviewers, or review comments so far.

Overall, the fix is clear, resolves a previously reported issue, and is consistent with existing project practices for similar test files. The change should help users building with CMake and testing enabled.

[james-sergeant-work]

@nigels-com Fancy seeing you here :) I was hoping the fix was as simple as this. Thanks - hope you're doing well.