7700 codes are relatively rare, so they are not the best test of the alert system.
They are indeed - I've had only two 7700s in the last 7 days (out of a total of around 29,000 flights/squawks recorded).
Top 10 squawks recorded over the last week:
CODE #
7000 558
0430 40
7710 40
7701 39
7706 39
0043 38
2252 38
7703 38
2226 37
2273 37
No contest, really, if you're looking for a the best squawk to use for testing purposes.