CI: Enable 32bit unit test (3)

[falbrechtskirchinger]

Third time's the charm.

Some of the decision-making logic for whether the 32bit test should be built has been moved into test.cmake and the code for constructing unit tests is now slightly less awkward. When only the 32bit unit test is requested, processing of the CMakeLists.txt is stopped early.

A single-header build has been added to CI.

[coveralls]

Coverage remained the same at 100.0% when pulling e752352 on falbrechtskirchinger:ci-enable-32bit-test into af34396 on nlohmann:develop.

[falbrechtskirchinger]

@fangq Can you help me out with a test case for this exception (possibly on 32bit only)?
https://github.com/falbrechtskirchinger/json/blob/9ba90f5b70fa9c05978e5ad6c8222f7bf6aaf04f/include/nlohmann/detail/input/binary_reader.hpp#L2083-L2087

I only need a std::vector with the byte sequence. Thanks!

Edit: I think I got it. Waiting for CI…

[nlohmann]

Looks good to me.

[nlohmann]

Thanks a lot!