tests: add test ensuring manual pages are shipped

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

tests/main/manpages/task.yaml

@@ -0,0 +1,9 @@
+summary: the essential manual pages are installed by the native package
+execute: |
+    for manpage in snap snap-confine snap-discard-ns ubuntu-core-launcher; do
+        if ! man --what $manpage; then
+            echo "Expected to see manual page for $manpage"
+            exit 1
+        fi
+    done
+# TODO: add manual pages for snapctl, snap-exec and snapd