tests/kernel/lifo/lifo_api: Don't reuse name for different thread stacks

A limitation on current HW shadow stack implementation for x86 is that
more than one thread stack can't share the same variable name, even if
they are static. To avoid skipping useful tests, for now, while this
limitation holds, change the name of the stack variable.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>