tests: squash Boost header deprecation warnings

tests: squash Boost header deprecation warnings

Boost 1.74.0 deprecates some functions that svgpp is using. Without squashing
these, the CMake build fails in CI:

  [ 86%] Building CXX object
    tests/CMakeFiles/test_common.dir/svgpp_context.cpp.o
  In file included from /usr/include/boost/config/header_deprecated.hpp:18,
                   from /usr/include/boost/detail/scoped_enum_emulation.hpp:15,
                   from /usr/include/svgpp/detail/namespace.hpp:13,
                   from /usr/include/svgpp/detail/attribute_name_to_id.hpp:11,
                   from
    /usr/include/svgpp/attribute_traversal/prioritized.hpp:12,
                   from
    /usr/include/svgpp/attribute_traversal/attribute_traversal.hpp:10,
                   from /usr/include/svgpp/document_traversal.hpp:10,
                   from /usr/include/svgpp/svgpp.hpp:1,
                   from /builds/graphviz/graphviz/tests/svgpp_context.h:7,
                   from /builds/graphviz/graphviz/tests/svgpp_context.cpp:8:
  /usr/include/boost/detail/scoped_enum_emulation.hpp:17:1: note: '#pragma
    message: This header is deprecated. Use <boost/core/scoped_enum.hpp>
    instead.'
     17 | BOOST_HEADER_DEPRECATED("<boost/core/scoped_enum.hpp>")
        | ^~~~~~~~~~~~~~~~~~~~~~~