selftests: livepatch: rename KLP_SYSFS_DIR to SYSFS_KLP_DIR

This naming makes more sense according to the directory structure.
Especially when we later add more paths.

Addtionally replace `/sys/kernel/livepatch` with `$SYSFS_KLP_DIR` in
the livepatch test files.

Signed-off-by: Michael Vetter <mvetter@suse.com>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Joe Lawrence <joe.lawrence@redhat.com>
Tested-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20241017200132.21946-2-mvetter@suse.com


[pmladek@suse.com: Fix corrupted substitution]
Signed-off-by: Petr Mladek <pmladek@suse.com>