cmake: Also build the test programs in BUILD_TESTING=Y mode

[nigels-com]

Extending the cmake build to also build the test programs.

Requires BUILD_TESTING mode and an installed boost that cmake can find via find_package.
Also requires using the tests/all ninja target.

Ideally this test coverage could be adapted as cmake tests, but this is the first step.

$ /opt/cmake-4.1.1-linux-x86_64/bin/cmake -G Ninja -DCMAKE_CXX_COMPILER=g++-14 .. -DBUILD_TESTING=Y
-- The CXX compiler identification is GNU 14.3.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/g++-14 - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using CMake version 4.1.1
-- Found Boost 1.90.0 at /opt/boost-1.90.0/lib/cmake/Boost-1.90.0
-- Requested configuration: REQUIRED COMPONENTS assert;config;core;optional;smart_ptr;throw_exception;utility
-- BoostConfig: find_package(boost_headers 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_headers 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_headers-1.90.0
-- BoostConfig: find_package(boost_assert 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_assert 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_assert-1.90.0
-- Found boost_config 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_config-1.90.0
-- BoostConfig: find_package(boost_config 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_config 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_config-1.90.0
-- BoostConfig: find_package(boost_core 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_core 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_core-1.90.0
-- Found boost_throw_exception 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_throw_exception-1.90.0
-- BoostConfig: find_package(boost_optional 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_optional 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_optional-1.90.0
-- Found boost_type_traits 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_type_traits-1.90.0
-- Found boost_static_assert 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_static_assert-1.90.0
-- BoostConfig: find_package(boost_smart_ptr 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_smart_ptr 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_smart_ptr-1.90.0
-- BoostConfig: find_package(boost_throw_exception 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_throw_exception 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_throw_exception-1.90.0
-- BoostConfig: find_package(boost_utility 1.90.0 EXACT CONFIG REQUIRED HINTS /opt/boost-1.90.0/lib/cmake)
-- Found boost_utility 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_utility-1.90.0
-- Found boost_io 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_io-1.90.0
-- Found boost_preprocessor 1.90.0 at /opt/boost-1.90.0/lib/cmake/boost_preprocessor-1.90.0
-- Configuring done (0.1s)
-- Generating done (0.0s)
-- Build files have been written to: /home/nigels/dev/format/build

[Flamefire]

You should rather use boost_test as this PR is missing e.g. the target creation, dependencies and actually running the tests. See e.g. Boost.Atomic for an example

[nigels-com]

Yes, that would be a good follow-up, agreed.

[Flamefire]

I'm not sure this change does what is intended: The test will not be built with this change unless you build the individual targets explicitly.
The super-project provides targets check and tests where the former depends on the latter and additionally runs the tests. This PR doesn't.

But as you seem to intent to build Boost.format against an installed Boost too, it is going to be harder and likely requires adding such targets manually.

[nigels-com]

It's as intended. Certainly could be taken further. Intended as a first step.