tests/gpio: fix test GPIO_INT_LEVEL bug

According to include/gpio.h, GPIO_INT_LEVEL is 0,
which means the branch to test GPIO_INT_LEVEL is
always false and GPIO_INT_LEVEL cases will always
pass.

This patch also fixes another minor bug in main.c.

Coverity-CID: 160074

Change-Id: I1fefa978e65bf801f244b4c2960cb87a26b0829e
Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>