tests: obj_core: Increase stack sizes

Increase the size of the thread stacks used by the obj_core tests
by CONFIG_TEST_EXTRA_STACK_SIZE bytes. This is useful to prevent
stack overflow/corruption when options such such as
    "--coverage --gcov-tool gcov"
are applied to twister runs.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>