ssh/tailssh: fix integration test

[irbekrm]

The new ssh integration test target is failing on main

$ make sshintegrationtest
# tailscale.com/ssh/tailssh [tailscale.com/ssh/tailssh.test]
ssh/tailssh/tailssh_integration_test.go:488:19: cannot use &testBackend{…} (value of type *testBackend) as ipnLocalBackend value in struct literal: *testBackend does not implement ipnLocalBackend (wrong type for method WhoIs)
                have WhoIs(netip.AddrPort) (tailcfg.NodeView, tailcfg.UserProfile, bool)
                want WhoIs(string, netip.AddrPort) (tailcfg.NodeView, tailcfg.UserProfile, bool)
make: *** [Makefile:113: sshintegrationtest] Error 1

presumably because of the order in which the PR with it and #12385 got merged.

Updates#cleanup

[oxtoacart]

Thanks @irbekrm !