dev-util/scons: 3.0.5-r1 fix test exit status processing

dev-util/scons: 3.0.5-r1 fix test exit status processing

1. The RESTRICT="test" is replaced with RESTRICT="!test? ( test )"
2. DEPEND "test" condition is added for dev-libs/libxml2[${PYTHON_USEDEP}]
   and dev-python/lxml[${PYTHON_USEDEP}] to provide similar test results
   for all python bindings for all tests there it is possible.
3. Test exit status processing of runtest.py results within python_test()
   is fixed to avoid failure if there are no FAILED tests.

The Scons runtest.py script returns 0 if all tests are passed
and returns 2 if there are any "no result" tests.
If failed tests exist then 1 returns.

Signed-off-by: Sergey Torokhov <torokhov_s_a@mail.ru>
Signed-off-by: Michał Górny <mgorny@gentoo.org>