libs/python/build/Jamfile.v2: remove needless <define> property.
libs/python/test/Jamfile.v2:
- add dynamically-linked embedding test
- fix builtin_converters test so it can work (BBv1 allowed the
duplication of main target names; BBv2 does not)
libs/python/test/import_.cpp: move some more of the Python code within
a handle_exception callback so at least we can better diagnose
failures.