docs: document native packaging tests

native/README.md

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+# Native packaging tests
+
+The openEuler packaging tests build and sign RPMs or require Linux namespaces.
+They run separately from `prove -r t` on a disposable host with the required
+native tools:
+
+```
+prove -v native/*.t
+```
+
+Use an ordinary user for RPM builds. `openeuler-power-inputs.t` exercises the
+whole build owner only on POWER with native Mock; its RPM admission checks also
+run on x86_64. When running that test as root, set `XCAT_TEST_BUILD_USER` to an
+unprivileged account for the fixture builds.
+
+The fixtures are command doubles at the downloader and Mock boundaries. The
+POWER Mock adapter uses the installed Python API directly to load generated
+configurations. It does not execute another Python interpreter.
+
+The Mock configuration test loads the installed Python API from a checked-in
+fixture and is kept outside the unit run.
+
+Inspect TAP skips: a successful exit does not qualify an unavailable native
+tool or architecture.