Remove boost_test_minimal

  CXX      tests/test-xmpformat.o
In file included from /usr/include/boost/config/header_deprecated.hpp:18,
                 from /usr/include/boost/test/minimal.hpp:34,
                 from ../../exempi/tests/test-xmpformat.cpp:39:
/usr/include/boost/test/minimal.hpp:35:1: note: ‘#pragma message: This header is deprecated. Use <boost/test/included/unit_test.hpp> instead.’
   35 | BOOST_HEADER_DEPRECATED( "<boost/test/included/unit_test.hpp>" )
      | ^~~~~~~~~~~~~~~~~~~~~~~
  CXXLD    testxmpformat

This should be replaced by something else.