You could turn it into a string with that format before writing it out. This could be done with a combination of a list comprehension and a join:
strs = [json.dumps(innerdict) for innerdict in d]
s = "[%s]" % ",\n".join(strs)
open('./testing.json','w').write(s)
In one line:
open('./testing.json','w').write("[%s]" % ",\n ".join(json.dumps(e) for e in d))
or:
open('./testing.json','w').write("[%s]" % ",\n ".join(map(json.dumps, d)))