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/test.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# Run the test scripts in the client container. Optional FILTER selects files
+# by substring, e.g. testenv/test.sh tunnel. The client is rebuilt first so the
+# tests always run the current source.
+set -euo pipefail
+cd "$(dirname "$0")"
+compose=(docker compose -f compose.yaml)
+if [ "${NO_BUILD:-}" != 1 ]; then
+  "${compose[@]}" build -q client
+  "${compose[@]}" up -d --wait client
+fi
+exec "${compose[@]}" exec -T client bash /tests/run.sh "${1:-}"