[evlatests] No PDif on ea25 at K-band

[Rick Perley]

    Antenna 25's noise diode is apparently not switching.  I see 
perfectly normal PSum values (which clearly show the change of system 
power as we go from source to source), but only noise on the PDif.  So 
either the diode is not on at all, or it is on and not switching.  I see 
normal PDif from this antenna at X and Ku bands, so it appears this is 
unique to K-band.