selftests/landlock: Don't create useless file layouts

Add and use a layout0 test fixture to not populate the tmpfs filesystem
if it is not required for tests: unknown_access_rights, proc_nsfs,
unpriv and max_layers.

This doesn't change these tests but it speeds up their setup and makes
them less prone to error. This prepare the ground for a next commit.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mickaël Salaün <[e-mail]>

tools/testing/selftests/landlock/fs_test.c

···
 	EXPECT_EQ(0, remove_path(TMP_DIR));
 }
 
+/* clang-format off */
+FIXTURE(layout0) {};
+/* clang-format on */
+
+FIXTURE_SETUP(layout0)
+{
+	prepare_layout(_metadata);
+}
+
+FIXTURE_TEARDOWN(layout0)
+{
+	cleanup_layout(_metadata);
+}
+
 static void create_layout1(struct __test_metadata *const _metadata)
 {
 	create_file(_metadata, file1_s1d1);
···
 	ASSERT_EQ(0, close(ruleset_fd));
 }
 
-TEST_F_FORK(layout1, unknown_access_rights)
+TEST_F_FORK(layout0, unknown_access_rights)
 {
 	__u64 access_mask;
 
···
 	}
 }
 
-TEST_F_FORK(layout1, proc_nsfs)
+TEST_F_FORK(layout0, proc_nsfs)
 {
 	const struct rule rules[] = {
 		{
···
 	ASSERT_EQ(0, close(path_beneath.parent_fd));
 }
 
-TEST_F_FORK(layout1, unpriv)
+TEST_F_FORK(layout0, unpriv)
 {
 	const struct rule rules[] = {
 		{
-			.path = dir_s1d2,
+			.path = TMP_DIR,
 			.access = ACCESS_RO,
 		},
 		{},
···
 	ASSERT_EQ(0, test_open(file1_s1d3, O_RDONLY));
 }
 
-TEST_F_FORK(layout1, max_layers)
+TEST_F_FORK(layout0, max_layers)
 {
 	int i, err;
 	const struct rule rules[] = {
 		{
-			.path = dir_s1d2,
+			.path = TMP_DIR,
 			.access = ACCESS_RO,
 		},
 		{},