tons of fixes for the test suite: docstrings must be unicode strings to run in Py3's doctest (obviously...)

--HG--
rename : tests/compile/withnogil.pyx => tests/run/withnogil.pyx

make it explicit in the generated source code what is an identifier string constant and what is a normal string constant that just happens to get interned

support running tests without running Cython to test the generated code under different Python versions (including Py3)