Add I2C testpoints / QWIIC connector on back board

[CRImier]

Right side of the phone (in the space between back board, front board and 40-pin connectors, where the "Pry apart here" markings are) seems to be a good spot where one could add some I2C sensors, i.e. an accelerometer/gyro, or a contactless temperature sensor. However, we should make this easier - to be exact, we need to add VCC (3.3V), GND, SDA and SCL testpoints there, in a spot where the user can solder to them with a soldering iron. We can even add a QWIIC connector there!