ptest: d.keys() is slow, use a list instead

ptest: d.keys() is slow, use a list instead

Unfortunately d.keys is extremely slow. Using a list in this case should be
fine since the addtask lines are immediately above the code and aren't
going to change often.

(From OE-Core rev: 5901189f0442cefc4d17623547f7358ffc2d37a6)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>