tests/kernel: Bump stack sizes for a few tests on qemu_x86

These tests had very small stacks, and the rbtree scheduler on
qemu_x86 (which does need a little extra stack room, though not much)
is bumping up against the limit.  Increase by ~128 bytes in most
cases.  In the case of the mbox_api test, there are other platforms
(which don't use the tree) which are right against the limit already
and will fail to link with a larger stack, so bump it for qemu_x86
only.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>