reduced provider test retry timeout
reduced provider test retry timeout

because the provider test calls a function that itself retries for up
to 75 seconds (plus execution time), we don't need to repeat 12 times -
that'll lead to 30 min + runtimes. so just retry 3 times, for a grand total
time of 4 minutes maximum. Only if our provider isn't giving delivery receipts

alphagov/notifications-functional-tests,alphagov/notifications-functional-tests