selftests/landlock: Add disconnected leafs and branch test suites

selftests/landlock: Add disconnected leafs and branch test suites

Test disconnected directories with two test suites
(layout4_disconnected_leafs and layout5_disconnected_branch) and 43
variants to cover the main corner cases.

These tests are complementary to the previous commit.

Add test_renameat() and test_exchangeat() helpers.

Test coverage for security/landlock is 92.1% of 1927 lines according to
LLVM 20.

Cc: Günther Noack <gnoack@google.com>
Cc: Song Liu <song@kernel.org>
Cc: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/20251128172200.760753-5-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>