test: show how purge removes ignored directories

Apparently the rust code path removes ignored directories
even though the pure code path doesn't.
Show this in tests.

tests/test-purge-ignored-directory.t

@@ -0,0 +1,13 b''
+skip ignored directories if -i or --all not specified
+
+  $ hg init t
+  $ cd t
+  $ echo 'ignored' > .hgignore
+  $ hg ci -qA -m init -d'2 0'
+  $ mkdir ignored
+
+The better behavior here is the non-rust behavior, which is to keep
+the directory and only delete it when -i or --all is given.
+
+  $ hg purge -v --no-confirm
+  removing directory ignored (known-bad-output rust !)