Update preshiprig test to check shipping preamp-ADC cable.

The b2 amp swap problem uncovered a hole in the JHU testing, where the final preamp-to-ADC cable only gets tested during final integration right before shipping, and we do not check for amp or ccd swaps. But we can.

At the start of the preshiprig test, clear the bias offsets stored in the FEE. This will show the stair step levels in the dummy CCDs, and thus point out any swapped amps within a CCD.

After checking read noise, run a second image acquisition with the integrator held open on one of the CCDs. This will show amps swapped between CCDs.

Needed for b3.