Surely a simpler way to test the system is to start the generator, synchronise to line, then increase the excitation voltage until the generator is nearly fully loaded. Quick, functionally realistic, value for money, and importantly is not a waste of our precious carbon resources. 

Many (maybe most) standby generators are specifically not allowed to synchronise, there are interlocks etc. But this may allow them to be run off line, not interrupting normal operations. Also very few installations will allow a full load test (or anything like it), it has to simulated with a load bank. When it's important enough the load bank will be permanently installed and switched in as required (e.g. data centres where big money is involved).

Many (maybe most) standby generators are specifically not allowed to synchronise, there are interlocks etc. But this may allow them to be run off line, not interrupting normal operations. Also very few installations will allow a full load test (or anything like it), it has to simulated with a load bank. When it's important enough the load bank will be permanently installed and switched in as required (e.g. data centres where big money is involved).