test: Fix pytest race in test_spawn_broken_pipe()

The static `time.sleep(0.1)` was racy: On slow architectures like Debian
mips [1] it repeatedly fails, and it even sometimes fails on GitHub
actions.

Keep writing until the transport closes, so that we are guaranteed to
eventually run into a BrokenPipeError.

Fixes #19469

[1] https://buildd.debian.org/status/logs.php?pkg=cockpit&ver=302-1&arch=mips64el

test/pytest/test_peer.py

@@ -189,8 +189,10 @@ def __init__(self, *, specific_error=False):
         async def do_connect_transport(self) -> None:
             transport = await self.spawn(['sh', '-c', 'exit 9'], ())
             assert isinstance(transport, SubprocessTransport)
-            time.sleep(0.1)  # sync! increase chance for process to end already
-            transport.write(b'abcdefg\n')
+            # The process will exit soon — try writing to it until a write fails.
+            while not transport.is_closing():
+                transport.write(b'x')
+                time.sleep(0.1)
             while transport.get_returncode() is None:
                 await asyncio.sleep(0.1)
             if self.specific_error: