Incomplete implementation of dummy audio manager can cause mismatches in headless mode

[Caball009]

*** IF ***
  Sound 'AirRaidSiren' has completed playing.
*** THEN ***
  Show 'Victorious' window and end game

The above script only works correctly if the implementation for AudioManager::getFileLengthMS returns the right value. Our dummy implementation AudioManagerDummy doesn't do that. Some custom maps with scripts like the above can mismatch in headless mode because of this.

Reproduction:

1. Use this map, it contains the above script: audio_manager_test.zip
2. Start a skirmish / multiplayer game, no interaction required.
3. Save the replay.
4. Check the replay inside the game. It should not mismatch.
5. Check the replay with headless mode, it should mismatch.