automaton: drop stopped children from the spawn listener

[KernelClint]

Automaton.spawn() runs a listener that accepts connections and starts a child automaton for each
one, tracking them in a clients list so they can be shut down with the listener.

Children are appended at
scapy/automaton.py:1041-1047
and never removed. Each new connection walks the entire list and calls destroy() again on every
child that has already stopped. Over a run of short-lived connections the list only grows, and the
work done per connection grows with it — the total is quadratic in the number of connections
served. The stopped children stay reachable too, so their memory is not reclaimed.

The change removes each stopped child once it has been destroyed, iterating a copy so removal
during iteration is safe:

-        for c in self.clients:
+        for c in list(self.clients):
             if not c[1].isRunning():
                 c[1].destroy()
+                self.clients.remove(c)

Live children are untouched, so listener shutdown still reaches all of them.

The added regression accepts several connections, stops them, and asserts the list holds only the
running children. Without the removal it fails.

Performance was measured on one computer, before and after the fix: accepting a connection while a
child is still running took 6,652.7 ns before and 6,669.2 ns after. Repeat runs moved by about 2%,
so that difference is smaller than the test can distinguish.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.63%. Comparing base (b3bbcc8) to head (b877242).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5129      +/-   ##
==========================================
- Coverage   80.63%   80.63%   -0.01%     
==========================================
  Files         390      390              
  Lines       96936    96937       +1     
==========================================
  Hits        78168    78168              
- Misses      18768    18769       +1     

Files with missing lines	Coverage Δ
scapy/automaton.py	80.15% <100.00%> (-0.18%)	⬇️

... and 7 files with indirect coverage changes

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