Move GIL's original test suites to test/legacy/ (#239)

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.

test/algorithm/Jamfile

@@ -1,6 +1,6 @@
 # Boost.GIL (Generic Image Library) - tests
 #
-# Copyright (c) 2018 Mateusz Loskot <mateusz at loskot dot net>
+# Copyright (c) 2018-2019 Mateusz Loskot <mateusz at loskot dot net>
 #
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or
@@ -8,11 +8,5 @@
 
 import testing ;
 
-project
-    : requirements
-    <include>$(BOOST_ROOT)
-    <include>..
-    ;
-
 run for_each_pixel.cpp ;
 run std_fill.cpp ;