SCons: Add tests option to enable or disable unit tests

[Xrayez]

Closes godotengine/godot-proposals#107.
Helps to workaround #40666.
Remake of #32387.

[RevoluPowered]

Can we edit the main/tests/scsub so that if tests=no nothing is done in that directory compilation wise?

[Xrayez]

Yeah this is already the case, but the check is within main/SCsub which calls into tests/SCsub, building with tests=no would prevent going into that dir.

if env["tests"]:
    SConscript("tests/SCsub")

That said, it's not possible to return from SCsub directly.

[Xrayez]

Added tests=yes to CI configs which run tests, because tests are disabled by default now, as suggested.

[akien-mga]

Thanks!