Boost CMake testing procedure doesn't work for Process

The [documented testing procedure](https://github.com/boostorg/cmake#testing-boost-with-cmake)

```
mkdir __build && cd __build
cmake -DBUILD_TESTING=ON -DBOOST_INCLUDE_LIBRARIES=process ..
cmake --build . --target tests
ctest --output-on-failure --no-tests=error
```

fails for Boost.Process, because the test executables aren't built by the target `tests`. BoostTest handles this automatically, but for tests declared manually, one needs to first declare the `tests` target if not present

```
if(NOT TARGET tests)
add_custom_target(tests)
endif()
```

and then for each test executable, use `add_dependencies(test my_test_executable)`.

(For this to keep working after the eventual fix, it might be a good idea to add it to CI.)

It's also good practice to make the test executables EXCLUDE_FOR_ALL, so that building the `tests` target builds them, but building the default target doesn't, but this step is not in principle required.