Two changes roughly halve the CPU time needed to build the unit tests
(clang 21: ~300 s -> ~150 s, GCC 16: ~750-820 s -> ~395-420 s, Debug):
- Move the C++14/17/20-dependent tests into separate
unit-<name>-cpp<N>.cpp files. A test file is built for every standard
whose JSON_HAS_CPP_<N> macro it mentions, so far whole large files were
rebuilt for C++14/17/20 because of a few #ifdef sections. The main files
are now built for C++11 only; the ci_test_*_cxx<N> jobs still build
every file for every standard.
- Add the CMake option JSON_TestUnityBuild (ON by default, OFF with
MinGW): compatible test files are compiled in batches as one
translation unit so they share the template instantiations of the
library. Each file keeps its own CTest test, which runs the batch
executable filtered to that file's test cases. The binary-format tests
form an explicit group; the rest is batched by JSON_TestUnityBatchSize.
Fix the name clashes that merging files exposed, document the rules for
test files in tests/README.md, and point CONTRIBUTING.md to it.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
