gtest: Qualify nmea_gps test with nmea_gps target
gtest: Qualify nmea_gps test with nmea_gps target

Change the unit test CMakeLists to only build the nmea_gps unit test if
the nmea_gps target exists.

Signed-off-by: Noel Eck <1e36ab6ededef40f3f16bdbc1b09eee72c5d08cf@intel.com>

g-vidal/upm,intel-iot-devkit/upm,intel-iot-devkit/upm,intel-iot-devkit/upm,stefan-andritoiu/upm,Propanu/upm,Propanu/upm,stefan-andritoiu/upm,intel-iot-devkit/upm,stefan-andritoiu/upm,stefan-andritoiu/upm,Propanu/upm,g-vidal/upm,g-vidal/upm,g-vidal/upm,Propanu/upm,Propanu/upm,intel-iot-devkit/upm,g-vidal/upm,g-vidal/upm,stefan-andritoiu/upm,stefan-andritoiu/upm,intel-iot-devkit/upm,Propanu/upm