Pebble reuses a valid authorization in a new order 50% of the time by
default, so the dns manual mode case was a coin flip: a reused
authorization left nothing for the TXT record to answer.
TEST_DNS_MANUAL=1 turns on le_test_dns_manual_renew, which answers the
dns-01 challenge through the pebble-challtestsrv that the compose setup
already runs.