gh-111569: Fix critical sections test on WebAssembly (GH-111897)

gh-111569: Fix critical sections test on WebAssembly (GH-111897)

This adds a macro `Py_CAN_START_THREADS` that corresponds to the Python
function `test.support.threading_helper.can_start_thread()`. WASI and
some Emscripten builds do not have a working pthread implementation.

This macro is used to guard the critical sections C API tests that
require a working threads implementation.