Replace tut with doctest

[daatsuka]

Summary

Replaces the legacy tut testing framework with doctest — a lightweight, single-header C++ testing framework.

Wallet: 2s37zauxhzbVM5SeLhLiryoGGaqcuv248ekRKU49rbWw

Changes

CMake infrastructure

• indra/cmake/Doctest.cmake: new file — creates a doctest INTERFACE library with the correct include root so #include <tut/tut.hpp> and #include "doctest/doctest.h" both resolve from any test target
• indra/cmake/LLAddBuildTest.cmake: link doctest interface to unit & integration test targets; removes hard-coded tut include path
• indra/test/CMakeLists.txt: add CMAKE_CURRENT_SOURCE_DIR to lltest include dirs; link doctest interface target

Test framework files

• indra/test/doctest/doctest.h: vendored doctest single-header (v2.4.11)
• indra/test/tut/tut.hpp: compatibility shim — #include <tut/tut.hpp> now silently redirects to lltut.h so all existing _tut.cpp files compile without modification
• indra/test/lltut.h: rewritten — wraps doctest, exposes legacy tut::failure, ensure, ensure_equals, and friends so existing test code needs zero changes
• indra/test/test.cpp: updated entry point — uses DOCTEST_CONFIG_IMPLEMENT + REGISTER_EXCEPTION_TRANSLATOR for tut::failure/tut::skip_exception
• indra/test/test.h: retains tut::sSourceDir declaration for compatibility

How it works

• Old tests using tut::test_group / tut::factory continue to work through the shim
• New tests can use TEST_CASE / SUBCASE / CHECK / REQUIRE directly
• tut::failure is registered as a doctest exception translator so throw-based assertions map to proper doctest failures

Closes #4445

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[daatsuka]

I have read the CLA Document and I hereby sign the CLA

[daatsuka]

recheck

[akleshchev]

@Geenz For the license is that note in the .h file sufficient? Finished viewer is not supposed to include code to run tests, but normally I would expect a note in 'about' floater.

Is it fine that 3p code gets integrated directly into viewer's branch? As it has own GH repo I would have expected either a build step to pull the repo/use the tool, submodule or pacakge, not what appears to be a full copy.

[daatsuka]

@akleshchev Thank you for the review — both points are valid.

License attribution: I've added the full doctest MIT license text to licenses-linux.txt, licenses-win32.txt, and licenses-mac.txt in 16ab9fb, so it will appear in the shipped binary's license file.

Integration method: You're right that vendoring the header directly isn't consistent with how the viewer manages 3p dependencies through autobuild. I'd be happy to restructure this — what would you prefer?

1. autobuild package — I can create a 3p-doctest repository following the same pattern as 3p-tut, package the header as a .tar.zst release, and switch Doctest.cmake to use_prebuilt_binary(doctest)
2. Submodule pointing to doctest/doctest upstream
3. Other approach you'd recommend

Let me know which direction works best for the project and I'll update the PR accordingly.

[akleshchev]

A '3p-doctest' is a safer option. But I suggest waiting for lindens to comment on that as they are needed for a repo either way. I don't have permissions to create a public repositiory and any such pacakge will have to live in secondlife's area of GH.

[akleshchev]

Also please note #4852.

[daatsuka]

Noted, thanks for the pointer to #4852. I've rebased and removed the bundled doctest.h along with Doctest.cmake, consolidating the test harness back into test.cpp and lltut.h (~7k lines dropped, ~600 added). Could you take a look when you get a chance and let me know if this aligns with the direction in that PR?