[boost] [smart_ptr] regression test case enhancement

I wrote two reference implementations for enable_shared_from_this.hpp, both are now in <https://svn.boost.org/svn/boost/sandbox/libs/smart_ptr/daniel_frey>. Both are tested and pass all regression tests - except for the non-enhanced version which obviously doesn't pass esft_constructor_test. The important thing is, that this test showed something else: The current tests without esft_constructor tests don't check who takes ownership of an enable_shared_from_this-derived object in case there are multiple simultaneous owners available. The attached patch checks that - and AFAICT it would fail with the 1.35.0 esft. (Both reference versions from above do pass). Should this be added? Regards, Daniel