tests: add an end-to-end test to show a bug in `visit_children_set`

Concretely, `rootfilesin` is completely broken with respect to
`visit_children_set` optimization.

tests/test-status.t

@@ -842,6 +842,12 b' Check the output'
   C clean
   C subdir/clean
 
+  $ hg status path:subdir
+  M subdir/modified
+  R subdir/removed
+  ! subdir/deleted
+  ? subdir/unknown
+
 FIXME: it's a bug in rhg that the status below is empty:
 
   $ hg status 'glob:subdir/*'
@@ -850,6 +856,14 b" FIXME: it's a bug in rhg that the status"
   ! subdir/deleted (no-rhg !)
   ? subdir/unknown (no-rhg !)
 
+FIXME: it's a bug (both in rhg and in Python) that the status below is wrong,
+in rhg it's empty, in Python it's missing the unknown file:
+
+  $ hg status rootfilesin:subdir
+  M subdir/modified (no-rhg !)
+  R subdir/removed (no-rhg !)
+  ! subdir/deleted (no-rhg !)
+
 Note: `hg status some-name` creates a patternmatcher which is not supported
 yet by the Rust implementation of status, but includematcher is supported.
 --include is used below for that reason