Document maintenance rules of `test/legacy/` content.

Motivation to move the `test/legacy/` is to:
* clarify status of those tests
* make it easier to run tests selectively
* separate new/upcoming feature-specific tests
  from those all-in-one tests.

Other changes:
Rename `gil_test_common.hpp` to `unit_test.hpp` to make
it clearer it acts as `boost/test/unit_test.hpp proxy`.
Remove `<include>$(BOOST_ROOT)` from `Jamfile`-s as unnecessary.