[boost] Leaner type traits tests

Those asking for leaner type traits tests will be pleased to know that they have been reworked so that they no longer depend on Boost.Test (unless you want it to by using --type-traits-unit-test as a command line arg to bjam). Unfortunately, while this does cut down on build dependencies, it doesn't reduce overall build times that much (which are mainly dominated by the time taken to instantiate the various templates). John.