tests: introduce doctest infra + migrate llcommon/llmath/llcorehttp subsets (ctest green) [relates to #4445]

[mayconbekkers]

Relates to #4445.

This PR adds a header-only doctest setup and migrates a first wave of unit tests while keeping the legacy TUT harness for suites not yet ported. The goal is to land a minimal, working baseline that the team can iterate on module by module.
This affects only unit-test targets behind LL_TESTS=ON; viewer runtime and packages are unchanged.

What’s included

• New doctest targets: llcommon_doctest, llmath_doctest, llcorehttp_doctest
• Shared helpers (LL_CHECK_* for floats, ranges, buffers, wide strings) + small Windows wide-string shim
• Deterministic llcorehttp fakes (zero-latency transport, monotonic clock, queued per-handle responses, redirects/retries/cancels) to keep tests network/IO-free
• Suites green locally:
  • llcommon_doctest: 100 cases
  • llmath_doctest: 93 cases
  • llcorehttp_doctest: 34 cases
• Quickstart doc: docs/testing/doctest_quickstart.md

How to run locally

# Enable tests
autobuild configure -c RelWithDebInfoOS -- -DLL_TESTS=ON

# Build (VS generator)
"C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\cmake.exe" --build build-vc170-64 --config RelWithDebInfo --target llcommon_doctest llmath_doctest llcorehttp_doctest -- /p:BuildProjectReferences=false

# Run
"C:\Program Files\Microsoft Visual Studio\2022\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin\ctest.exe" -C RelWithDebInfo -R "(llcommon_doctest|llmath_doctest|llcorehttp_doctest)" -V --test-dir build-vc170-64

Scope / non-goals

Does not change viewer runtime or packaging.
TUT remains for non-migrated suites; removal will happen after follow-ups when coverage is sufficient.

Proposed follow-ups

Continue migrating remaining suites per module (llcommon -> llmath -> llcorehttp -> newview/test).
Once coverage is high enough, remove Tut.cmake glue and clean up LLAddBuildTest.cmake usage.

[github-actions]

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[mayconbekkers]

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

[Ansariel]

What are the changes to .gitignore and .gitattributes for? They seem in no relation to doctest and counterproductive,

[mayconbekkers]

@Ansariel Thanks for the note! I’ve dropped the unrelated .gitignore/.gitattributes edits so this PR stays focused on doctest.
Also added the standard $LicenseInfo headers to the new test files and a third-party MIT notice for indra/extern/doctest/doctest.h.
pre-commit run -a passes locally; once workflows are approved for this fork, CI should reflect that.
Happy to adjust anything else if the maintainers prefer.