Add test_long_kernel

On Python 3.12, this provokes a stack overflow in the scheduler. It is
not quite clear why that's the case; pure-Python recursion even with
generators seems to respond well to setrecursionlimit():

```py
def f(n):
    if n:
        yield from f(n-1)
    else:
        yield 5

import sys
sys.setrecursionlimit(3500)
print(list(f(3400)))
```

That said, there have been [behavior](https://github.com/python/cpython/pull/96510)
[changes](https://github.com/python/cpython/issues/112215)
in Py3.12 in this regard, but it is not clear what exactly
about Loopy's behavior makes it fall into the 'bad' case.