At what test frequency should electrolytic capacitors be measured?
The answer to a slightly different question, which is "what is the standard measurement parameters for electrolytic capacitor capacitance?" is as follows:

Modern electrolytic capacitors use a standardized measurement method to determine their capacitance.  This method is 20°C and 0.5VAC at either 100 or 120Hz (which is derived from the traditional common usage of electrolytic caps, which was to smooth the output of a bridge rectifier.  That ripple is going to be 2*50Hz or 2*60Hz, depending on the country).  The exception is photoflash capacitors, which typically have their DC capacitance measured. 

Virtually all electrolytic capacitors will cease being capacitive at all before 100kHz, and beyond that, will be inductive.  This is universal to all electrolytic capacitors.  Generally, an electrolytic capacitor will gain about 10% capacitance if the DC capacitance is measured.  It will lose about 10% by 1kHz (or more in the case of your capacitors - but they say they are designed for DC and ripple applications, so they are probably not optimized for anything above 120Hz). 

But to actually answer your question, "at what test frequency should electrolytic capacitors be measured?" They should be measured at the frequency of interest for your application.  That's the frequency they should be measured at.  Why would you ever measure their capacitance at anything except the frequency of most interest to your planned use?  

Which makes me wonder - why are you even doing this?  It sounds like you are trying to verify if a capacitor is within spec.  There is no reason to do that however, and you are just wasting your time if you are.  

The failure mode of electrolytics does not involve a reduction in capacitance, so measuring it and comparing that value to what it was designed to have is not useful.

Beyond that, virtually every electrolytic capacitor, including those russian ones, have pretty awful tolerance.  They can vary from -20% to +80% the rated value.  So even if loss of capacitance WAS a valid way to determine if an electrolytic had gone bad or not (which it isn't), then a capacitor that reads the right value might have began life at +80% that value, and has lost nearly half of its capacitance, but you'd never know.  Not that this ever happens though.

The failure mode of electolytic caps is that their electrolyte dries out.  This does not have a meaningful impact on their measured capacitance.  What it does do, however, is cause ever increasing ESR, until eventually the ESR is so high that they are no longer useful as capacitors.  Even then, they will have the 'right' capacitance.  So I can think of nothing useful being accomplished by verifying the actual capacitance value.  

Now, if you don't know the value of a capacitor, because the label has fallen off, the lettering rubbed off, or whatever, then that's when you can do something useful by measuring the capacitance.  Or if you need to verify the capacitance at a specific frequency of interest.