[boost] [detail][lightweight_test.hpp] Proposal to add BOOST_TEST_THROWS to boost/detail/lightweight_test.hpp

Hi, In my tests I use boost/detail/lightweight_test.hpp as it has nearly all I need to test my libraries with very little overhead. However I needed a simple utility to check if an expression throws an exception and I think it can be added to this header with very little code. See attached patch. Do you find this a good idea? Ok to commit? Best, Ion