refactor: Move TestSink to src/test_helpers

[mvadari]

Summary

Follow-up to a review comment on #7012: #7012 (comment)

That PR introduces src/test_helpers/ as the home for test helpers shared across the test and benchmark binaries (starting with TempDir.h, moved out of include/xrpl/beast/utility/temp_dir.h). TestSink should be moved there as well.

Current state

TestSink lives in the gtest-only helper directory:

• src/tests/libxrpl/helpers/TestSink.h
• src/tests/libxrpl/helpers/TestSink.cpp

and is included as <helpers/TestSink.h> by, among others:

• src/tests/libxrpl/basics/KeyCache.cpp
• src/tests/libxrpl/basics/TaggedCache.cpp
• src/tests/libxrpl/consensus/Consensus.cpp
• src/tests/libxrpl/net/HTTPClient.cpp
• src/tests/libxrpl/nodestore/Database.cpp
• src/tests/libxrpl/peerfinder/Livecache.cpp
• src/tests/libxrpl/shamap/SHAMap.cpp

Because it sits under src/tests/libxrpl/helpers, it is only reachable from the xrpl_tests target, so the older src/test suites cannot use it.

Proposed change

1. Move TestSink.h/TestSink.cpp to src/test_helpers/.
2. Update the includes to <test_helpers/TestSink.h> and adjust the xrpl_tests source list in src/tests/libxrpl/CMakeLists.txt.
3. Make sure the isolate_headers/include wiring added in refactor: Replace boost::filesystem with std::filesystem across the codebase #7012 exposes it to the other test and benchmark targets that need it.

Optional cleanup

There are two unrelated, locally-defined TestSink classes that could be consolidated onto the shared one once it is generally reachable:

• src/test/beast/beast_Journal_test.cpp:11
• src/test/server/Server_test.cpp:79

Note these have different semantics (severity threshold, suite-reporting), so consolidation may not be a straight substitution.

Blocked on

#7012 landing first, since src/test_helpers/ does not exist on develop yet.