[roll] Roll fuchsia [wlan] Simplify SshConnection lifecycle in IPerfServerOverSsh

During the Antlion to Lacewing migration, legacy code was ported
where close_ssh() set self._ssh_session to None. In Lacewing,
SshConnection lazily manages ControlMaster processes and reconnects
on demand; nulling it broke reconnection (AssertionError in
_cleanup_iperf_port) and reset stateful command wrappers like
LinuxJournalctlCommand.

OpenWrt and Whirlwind (legacy) testbeds:
- Retain non-nullable self._ssh_session across close_ssh() calls.
- Remove redundant null assertions and _get_ssh() re-instantiations.
- Add unit tests verifying session reuse after close_ssh().

Whirlwind (legacy) testbeds:
- Preserve LinuxJournalctlCommand cursor in get_systemd_journal()
  (OpenWrt does not use systemd/journalctl).
- Add unit test verifying journalctl state preservation.

Multiply: iperf_server_test
Original-Original-Bug: 567209831, 567210166
Test: fx test iperf_server_test
Test: ThroughputTest (Astro): http://cr-buildbucket.appspot.com/build/8669021211617980945
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1863995
Original-Original-Revision: 878906e37f971d5fe6429f9b12ea1c31211b9ff2
GitOrigin-RevId: c7dd83a20b51f43c2d83e6c0abef4bf6202cee49
Change-Id: I4a3a0ad328f1572ab47d11d2402fca1c5e3f2af1
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1877607
Cr-Commit-Position: refs/heads/main@{#204954}