tests: threads: fix kernel/thread_stack test

Fix the broken logic in the kernel/thread_stack test
The modified test should do direct read & write from estimated stack
pointer to highest address in the stack buffer.
Previously this test was start from lowest address in the stack
which would trigger exception of hardware stack checking scheme
violation on ARC boards and other targets with hardware stack
overflow detection.

Signed-off-by: Yuguo Zou <yuguo.zou@synopsys.com>