test: add a local Docker test environment for all features

testenv/ starts two real NetBox 4.7 instances (v2 and v1 token), SSH
targets on public, mgmt, and prod networks, a jump host, an IPv6-only
host, a static host, and a client with ssh, sftp, scp, tmux, zellij,
bash, zsh, and fish. A seed script fills NetBox through the REST API.

141 end-to-end checks cover cache and refresh filters, connecting,
all resolver strategies, multiple instances, shortcuts and rewrites,
SSH options and rules, static hosts, sftp/scp incl. IPv6 and jump
hosts, tunnels, local notes and NetBox comments, completion and shell
hooks, the TUI (driven through tmux), the configurator, and uninstall.

Local only: testenv/up.sh, test.sh, shell.sh, down.sh. Not part of
CI/CD.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

testenv/targets/sshd_config

@@ -0,0 +1,18 @@
+# Test targets accept the generated test key only. Port 2222 tests ssh.default_port.
+Port 22
+Port 2222
+ListenAddress 0.0.0.0
+ListenAddress ::
+HostKey /etc/ssh/ssh_host_ed25519_key
+PermitRootLogin no
+PasswordAuthentication no
+KbdInteractiveAuthentication no
+PubkeyAuthentication yes
+AuthorizedKeysFile .ssh/authorized_keys
+AllowTcpForwarding yes
+AllowAgentForwarding yes
+GatewayPorts no
+PermitTunnel no
+X11Forwarding no
+PrintMotd no
+Subsystem sftp /usr/lib/ssh/sftp-server