tests/gpio: don't call risk function

According to Coverity, rand() is a risk function.
So we use another method to generate random value.

Coverity-CID: 160070

Change-Id: Icb7e06cd43cb8bade6596cc37e9d04bfc59e1de7
Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>
2 files changed