generic: Add tests for filename casefolding feature

generic: Add tests for filename casefolding feature

This new test implements verification for the per-directory
case-insensitive feature, as supported by the reference
implementation in Ext4.

Signed-off-by: Lakshmipathi.G <lakshmipathi.ganapathi@collabora.co.uk>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
  [Rewrite to support feature design]
  [Refactor to simplify implementation]
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>