tests: gpio: add initial state of GPIO set bits

When a GPIO is 1 and port_set_bits_raw_and_verify function does
not set that bit, it will still be 1. So calculate the expected
value only by the parameter of port_set_bits_raw_and_verify is
not enough. A proper way is to clear all the GPIO bit fist and
then do the test.
Fixes: #49714

Signed-off-by: Hu Zhenyu <zhenyu.hu@intel.com>