selftests/open_tree: add OPEN_TREE_NAMESPACE tests

Add tests for OPEN_TREE_NAMESPACE.

Link: https://patch.msgid.link/20251229-work-empty-namespace-v1-2-bfb24c7b061f@kernel.org
Tested-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/filesystems/open_tree_ns/.gitignore

@@ -0,0 +1 @@
+open_tree_ns_test

tools/testing/selftests/filesystems/open_tree_ns/Makefile

@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_GEN_PROGS := open_tree_ns_test
+
+CFLAGS := -Wall -Werror -g $(KHDR_INCLUDES)
+LDLIBS := -lcap
+
+include ../../lib.mk
+
+$(OUTPUT)/open_tree_ns_test: open_tree_ns_test.c ../utils.c
+	$(CC) $(CFLAGS) -o $@ $^ $(LDLIBS)