overlay: add tests for casefolded layers

overlay: add tests for casefolded layers

Overalyfs did not allow mounting layers with casefold capable fs
until kernel v6.17 and did not allow casefold enabled layers
until kernel v6.18.

Since kernel v6.18, overalyfs allows this kind of setups,
as long as the layers have consistent encoding and all the directories
in the subtree have consistent casefolding.

Create test cases for the following scenarios:
- Mounting overlayfs with casefold disabled
- Mounting overlayfs with casefold enabled
- Lookup subdir in overlayfs with mismatch casefold to parent dir
- Change casefold of underlying subdir while overalyfs is mounted
- Mounting overlayfs with strict enconding, but casefold disabled
- Mounting overlayfs with strict enconding casefold enabled
- Mounting overlayfs with layers with inconsistent UTF8 version

Co-developed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>