test: give systemd chance to actually start the unit

In certain cases the expected enqueue-start-replace-continue
sequence would end up as enqueue-replace-start-continue which causes
unexpected fails even though the serialization/deserialization part
works as expected. As we can't use `--wait` in this case, let's give
sysetmd a second to actually start the unit before replacing it with
another one.

Also, switch from the single-letter test output to a bit verbose format.

Fixes: #14632