Intermittent errors with tests on x86_64 / amd64

Start with tests/test_pane.py:113 and reproduce the failure in the openSUSE Tumbleweed x86_64 environment using tmux-3.3a-2.2. Compare the captured pane output around test_capture_pane_start and establish a reliable test result for the intermittent behavior.

When packaging this for openSUSE Tumbleweed, we get intermittent build errors due to failing tests.

Today this one errored out:

[  126s] =================================== FAILURES ===================================
[  126s] ___________________________ test_capture_pane_start ____________________________
[  126s] 
[  126s] session = Session($1 libtmux_udfg_nz8)
[  126s] 
[  126s]     def test_capture_pane_start(session: Session) -> None:
[  126s]         env = shutil.which("env")
[  126s]         assert env is not None, "Cannot find usable `env` in PATH."
[  126s]     
[  126s]         session.new_window(
[  126s]             attach=True,
[  126s]             window_name="capture_pane_start",
[  126s]             window_shell=f"{env} PS1='$ ' sh",
[  126s]         )
[  126s]         pane = session.attached_window.attached_pane
[  126s]         assert pane is not None
[  126s]         pane_contents = "\n".join(pane.capture_pane())
[  126s]         assert pane_contents == "$"
[  126s]         pane.send_keys(r'printf "%s"', literal=True, suppress_history=False)
[  126s]         pane_contents = "\n".join(pane.capture_pane())
[  126s]         assert pane_contents == '$ printf "%s"\n$'
[  126s]         pane.send_keys("clear -x", literal=True, suppress_history=False)
[  126s]         pane_contents = "\n".join(pane.capture_pane())
[  126s] >       assert pane_contents == "$"
[  126s] E       assert '$ printf "%s"\n$ clear -x' == '$'
[  126s] E         - $
[  126s] E         + $ printf "%s"
[  126s] E         + $ clear -x
[  126s] 
[  126s] tests/test_pane.py:113: AssertionError
[  126s] =============================== warnings summary ===============================
[...]

tmux version was tmux-3.3a-2.2.