Adds container_running/container_health/wait_for_container/start_db
helpers and cmd_full(), plus tests. Fixed a delimiter bug in the two
new tests' sed scripts (the literal `${TARGET#/}` replacement text
collided with `#` as the sed delimiter) and relaxed the dry-run
no-external-calls assertion to allow the legitimate borg-list call
from resolve_archive(), mirroring the existing db dry-run test.
kbe