tests: i2c_slave_api: Add missing testcase.yaml
tests: i2c_slave_api: Add missing testcase.yaml

The i2c_slave_api test was missing a testcase.yaml and therefore not
getting built by CI.

Signed-off-by: Maureen Helm <87a696db5547fa477bda9f9e0f00013c839c14d3@nxp.com>

nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr