Add scripts/testenv.sh to set up a test machine

On a fresh Debian/Ubuntu machine it installs BIND and the build tools
(Go from go.dev), writes a hand-made style configuration with internal
and external views, a split-horizon zone (example.com or
JARL_TEST_ZONE) and a reverse zone, starts named and prints the steps
to install Jarl and adopt the configuration.

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

docs/development.md

@@ -114,6 +114,36 @@ ACME hooks, audit) and checks the answers with `dig`. `KEEP=1` leaves named
 and jarld running afterwards. The output says where; set `JARL_CLI_CONFIG`
 to that directory's `cli.yaml` to keep using the CLI against it.
 
+### A test machine
+
+`scripts/testenv.sh` turns a fresh Debian/Ubuntu VM or container into a
+server to try Jarl on, the way a user would. Run it as root from the
+repository:
+
+```sh
+sudo scripts/testenv.sh                         # zone example.com
+sudo JARL_TEST_ZONE=lab.test scripts/testenv.sh # another zone
+```
+
+It installs BIND 9, make, git, curl and Go (from go.dev, at the version in
+`go.mod`, unless a new enough one is installed). It then writes a typical
+hand-made configuration for Jarl to import: an ACL, options with logging,
+an `internal` view (this machine's networks and the private ranges, with
+recursion) and an `external` view (everyone else, authoritative only).
+The zone is in both views with different answers. There is also a reverse
+zone for the machine's /24. The zone files in `/etc/bind/zones` use
+comments, `$TTL`, `$ORIGIN`, `$GENERATE`, legacy `type master` and most
+record types. The script checks the result with `named-checkconf -z`,
+(re)starts named (in the background where there is no systemd), and asks
+named a few questions with `dig`. At the end it prints the steps to build
+and install the packages, then run `init`, `import` and `adopt`.
+
+`JARL_TEST_VIEWS=no` leaves out the views, and `JARL_TEST_IP` sets the
+machine's address (default: the address of the default route). The old
+`/etc/bind` is saved in `/root/bind-before-testenv-<time>.tgz`. Once Jarl
+has adopted the configuration, running the script again needs
+`JARL_TEST_FORCE=1`. Debian 13 has BIND 9.20; Ubuntu has it from 25.04.
+
 ### The web UI
 
 The UI is plain JavaScript modules under `internal/web/static/`, with no