docs(test): document Ubuntu image fixture boundaries

Describe the native test prerequisites, external command doubles and the image-building behavior these fixtures do not qualify.

xCAT-test/native/README.md

@@ -16,6 +16,34 @@ It can run as root or an ordinary user with user namespaces enabled. This case
 checks service policy and ordering; it does not boot or reinstall an OS.
 The client steps cannot access the source tree or the server postscripts directory.
 
+## Ubuntu media and image fixtures
+
+The pull-request workflow runs these four tests explicitly:
+
+```
+prove xCAT-test/native/ubuntu_live_media_guardrails.t \
+      xCAT-test/native/ubuntu_genimage_debootstrap_arch.t \
+      xCAT-test/native/ubuntu_genimage_apt_mirror.t \
+      xCAT-test/native/ubuntu_genimage_resolver_libs.t
+```
+
+They require Linux user namespaces, Bubblewrap, cpio, gzip, GNU timeout and
+the system Perl in `/usr/bin` or `/bin`. Install the source-tree Perl dependencies,
+including File::Slurper, DBI and DBD::SQLite, for that interpreter. Tools must be
+in `/usr/bin`, `/bin`, `/usr/sbin` or `/sbin`. Run the tests as an ordinary user.
+Missing Linux prerequisites fail the tests; other operating systems skip them.
+
+The fixtures execute the checkout's unchanged Debian plugin and Ubuntu genimage
+script with private filesystems and no network access. Media copying, database
+updates, the image's APT sources.list and initrd archives are real. Decompression
+and archive inspection are real too. The debootstrap command records its arguments;
+the architecture test stops at its deliberate failure. The mirror test lets it
+complete, replaces mount commands and stops at a deliberate apt-get update failure.
+The initrd fixture supplies chroot ldd and depmod results. These tests do not install
+packages, execute target-architecture binaries or prove that the generated image boots.
+
+## Other native tests
+
 Run the package and unprivileged namespace cases as an ordinary user:
 
 ```