tests: add sshd_config.d include regression test

sshd on XCP-ng hosts has silently ignored /etc/ssh/sshd_config.d/*.conf
drop-in files due to a missing Include directive in the packaged
sshd_config. Add a quick host-only test that drops a harmless Banner
directive in the directory and checks it shows up in `sshd -T`'s
effective configuration.

Signed-off-by: Lucas RAVAGNIER <lucas.ravagnier@vates.tech>

Author: Lucas RAVAGNIER

tests/system/test_sshd_config_include.py

@@ -0,0 +1,28 @@
+from lib.common import Defer
+from lib.host import Host
+
+# Regression test for sshd silently ignoring /etc/ssh/sshd_config.d/*.conf
+# drop-in files (missing "Include" directive in the packaged sshd_config).
+#
+# A drop-in file is created, setting a directive (a pre-auth Banner), that
+# isn't set anywhere else. `sshd -T` dumps the configuration as sshd itself
+# would apply it, so finding the directive there proves the drop-in directory
+# is actually included.
+#
+# Requirements:
+# - an XCP-ng host (--hosts) >= 8.2
+
+SSHD_CONFIG_D = "/etc/ssh/sshd_config.d"
+
+def test_sshd_config_d_is_included(host: Host, defer: Defer) -> None:
+    marker = host.ssh('mktemp')
+    dropin = f"{SSHD_CONFIG_D}/99-xcp-ng-tests-include-check.conf"
+
+    host.ssh(f"echo 'Banner {marker}' > {dropin}")
+    defer(lambda: host.ssh(f"rm -f {dropin}"))
+
+    effective_config = host.ssh("/usr/sbin/sshd -T")
+    assert f"banner {marker}" in effective_config, (
+        f"drop-in {dropin} was not picked up by sshd -T: "
+        f"{SSHD_CONFIG_D} seems to be ignored"
+    )