[Cython] Test runner changes for Python/PyPy backend

[Stefan Behnel]

Hi,

Romain wants to start writing tests for his Python/ctypes backend branch, 
so we need to extend the test runner.

We'll eventually want to merge his branch back into normal mainline, so I 
suggest actually just running the complete test suite in a new, separate 
configuration, just like we do with C and C++ now. So we'd have a switch 
"--no-ctypes" that disables these tests, and (I assume) a new test class 
(inheriting from the normal compile test case) that properly runs them in 
plain CPython. If the implementation becomes PyPy specific, "--no-pypy" 
will make more sense, but it currently appears to target ctypes in general.

I'm also considering making the backend selection a positive opt-in, i.e. 
switch from "--no-cpp" and friends to "--backends=c,cpp,ctypes". That would 
have avoided the future configuration change in all existing Jenkins test 
jobs (which will then need to disable the new backend tests).

Does that fit everyone's expectations?

There's one problem I see: the ctypes backend will likely have most 
optimisations disabled, so many (most?) of the test tree assertions will 
make no sense. I think it's best to just keep that out of the backend 
specific pipeline completely for now. We have the C/C++ test modes for that.

And, obviously, switching from duplicating to triplicating the test cases 
will substantially increase the time for a complete test run, although a 
lot of the running time is currently spent in the C compiler more than the 
actual test code runs.

Stefan