ztest: apply extra stack size to ztest stack

The directive exists for platforms that require more stack space than
usual. However, it wasn't being applied to the ztest thread stack
which ztest-enabled cases are run on.

Fixes numerous failures on xtensa simulator targets.

Change-Id: Iafa84de002421f03729c0f0cdeefdea51842ae32
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>