I should add as I have posted before a VFD takes the AC changes it to DC which feeds the inverter section that changes it back to AC, the input to the VFD can be designed for your 50hz AC because it is nothing but a AC to DC conversion like a battery charger sort of speak, then this DC feeds an inverter that would create the 60hz you are looking for.
A simpler method would be to just let the 50hz AC from your inverter supply a DC power supply that would in turn feed a 60hz grid tie inverter which would be more common to you, this way you would be loading both the array and the 50hz inverter and the rest will be nothing more then a load that would have some usefulness in putting some power back into the grid instead of going to waste, yes there would be some loss so the power put back into the grid would be less then it would if the 50hz could be put into the grid, but at the same time it would provide a load that could be tested and recorded at the 50hz output of the inverter your are testing that would provide useful info.
The only thing you wouldn't be able to test is the 50hz inverters abilities to detect the grid as it would have to be run as an off grid inverter using the above load, or any load, which you have that problem now with just using a resistor/heater bank as a load.
Simulating a 50hz grid frequency at the output of the inverter is not an easy task as this is an input to the output of the inverter that the inverter looks for, and the load has to supply this 50hz signwave which most loads would not supply, and there lies your problem as you would have to have another 50hz source that would mimic the grid, it could be done but not in an easy way that would also provide a load for the inverter to dump into without the inverter changing the frequency of the simulated grid as it would not be stiff enough, a 60hz generator running at a reduced speed for 50hz would provide this reference but would be a more costly test method and as a load is applied the generator would fluctuate in speed as the load is applied and turned off this will cause a change in frequency as the RPM's of the generator changes.
Maybe a generator with a large flywheel might work, but it would have to be tested, FYI any two pole generator(alternator) ran at 3000 RPM's will produce 50hz, 3600 RPM's for 60hz.
Ok how about this, use a 220 volt 5kw motor powered from the inverter, that runs two alternators one at 3000 RPM's and one at 3600 RPMs, have the output of the inverter also connected to the 3000 RPM alternator, and use the 3600 RPM alternator to feed other loads, this at least would provide both things you would be looking for a load and a source to simulate the grid, the motor pulleys as well as the alternators would have to be sized to have the alternators run at the correct RPM's.
Ok maybe I'm over thinking this LOL