tests: schedule_api: use stack array extern macro

The stack array tstacks was declared in the header file using
the same macro which defines the same stack array but with
an added "extern" in front. This macro adds alignment and section
attribute which are actually not the same as the actual stack array
defined in main.c. The section name used in the section attribute
contains the file name where the stack array is defined or extern
declared. So the same symbol, in this case z_interrupt_stacks, has
different attributes in two places, and GCC 11 starts to complain
about this. So use the newly introduced macro to extern declare
the stack array without adding/replacing any symbol attributes.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>