[evlatests] 75 MHz tuning

[Ken Sowinski]

We want to avoid placing the 75 MHz band in subband 0 of the
correlator to avoid the roll off that has been seen.  Choosing
a negative signed sum of the LOs will accomplish this.  However
the station board uses a negative SSLO passed to it in a delay
packet as a lower sideband flag.

I have created a 4 band test file which uses subband zero to
avoid this, but we probably want to consider a more flexible
solution.