SCons: Build tests/ and main/ in cloned environments

[akien-mga]

Allows switching tests=yes/no and rebuilding only tests and main,
instead of the whole engine.

Supersedes #40725.

[Xrayez]

Yeah I don't see where we'd need to use TESTS_ENABLED except for main, and we don't embed test code in other places (if that's a feature of doctest), perhaps it could be hypothetically used for modules #40720 but that's far fetched as you say. 🙂

[akien-mga]

perhaps it could be hypothetically used for modules #40720 but that's far fetched as you say.

Well I do think #40720 is a good change, but I don't think tests written for modules need to have access to TESTS_ENABLED, they'll be called by tests/test_main.cpp which has it defined already. Or do I misunderstand how they work?

[Xrayez]

Yeah likely modules don't need to know about TESTS_ENABLED anyways. Collecting those headers seems to be enough to make it work. What's left is to port those tests with sources #40659.