testsuite: Replace ffitestcxx.h with ffitest.h

ffitest.h contains a superset of the functionality of ffitestcxx.h;
make the C++ tests include ffitest.h instead, and remove ffitestcxx.h.
testsuite: Add ABIs to the test matrix; unify tests across ABIs

This eliminates all the *_win32.c tests in favor of the tests they were
branched from, and expands test coverage to run many more tests on
stdcall, thiscall, and fastcall.

This same mechanism also supports testing any other target that has
multiple ABIs.