UB comparing unrelated pointers in priority_order

Detected by asan with gcc10:

==863==ERROR: AddressSanitizer: invalid-pointer-pair: 0x7ffe21c831b0 0x00000c26f470
    #0 0x2688702 in boost::unit_test::framework::state::priority_order::operator()(boost::unit_test::test_observer*, boost::unit_test::test_observer*) const libs/test/include/boost/test/impl/framework.ipp:865
    boostorg#1 0x2688702 in std::_Rb_tree<boost::unit_test::test_observer*, boost::unit_test::test_observer*, std::_Identity<boost::unit_test::test_observer*>, boost::unit_test::framework::state::priority_order, std::allocator<boost::unit_test::test_observer*> >::_M_get_insert_unique_pos(boost::unit_test::test_observer* const&) /opt/gcc-10.2.0/include/c++/10.2.0/bits/stl_tree.h:2101
    boostorg#2 0x268aafb in std::pair<std::_Rb_tree_iterator<boost::unit_test::test_observer*>, bool> std::_Rb_tree<boost::unit_test::test_observer*, boost::unit_test::test_observer*, std::_Identity<boost::unit_test::test_observer*>, boost::unit_test::framework::state::priority_order, std::allocator<boost::unit_test::test_observer*> >::_M_insert_unique<boost::unit_test::test_observer*>(boost::unit_test::test_observer*&&) /opt/gcc-10.2.0/include/c++/10.2.0/bits/stl_tree.h:2154
    boostorg#3 0x2614ac4 in std::set<boost::unit_test::test_observer*, boost::unit_test::framework::state::priority_order, std::allocator<boost::unit_test::test_observer*> >::insert(boost::unit_test::test_observer*&&) /opt/gcc-10.2.0/include/c++/10.2.0/bits/stl_set.h:521
    boostorg#4 0x2614ac4 in boost::unit_test::framework::register_observer(boost::unit_test::test_observer&) libs/test/include/boost/test/impl/framework.ipp:1380
    boostorg#5 0x2642b56 in boost::unit_test::framework::register_observer_helper::register_obs() libs/test/include/boost/test/impl/framework.ipp:1627
    boostorg#6 0x2642b56 in boost::unit_test::framework::run(unsigned long, bool) libs/test/include/boost/test/impl/framework.ipp:1731
    boostorg#7 0x27f6c77 in boost::unit_test::unit_test_main(boost::unit_test::test_suite* (*)(int, char**), int, char**) libs/test/include/boost/test/impl/unit_test_main.ipp:250
    boostorg#8 0x27f9495 in main libs/test/include/boost/test/impl/unit_test_main.ipp:306
    boostorg#9 0x7f8d8b3c1554 in __libc_start_main ../csu/libc-start.c:266
    boostorg#10 0x568e0d  (test+0x568e0d)

include/boost/test/impl/framework.ipp

@@ -862,7 +862,8 @@ public:
     struct priority_order {
         bool operator()( test_observer* lhs, test_observer* rhs ) const
         {
-            return (lhs->priority() < rhs->priority()) || ((lhs->priority() == rhs->priority()) && (lhs < rhs));
+            return (lhs->priority() < rhs->priority()) || ((lhs->priority() == rhs->priority()) &&
+                std::less<test_observer*>()(lhs, rhs));
         }
     };