The new tag takes ids instead of needing coordinates. So instead of storing
units or hardcoding values, an attack can be done like this:
```
[test_do_attack_by_id]
attacker=alice
defender=bob
weapon=0
[/test_do_attack_by_id]
```
Add a simpler unit for unit tests than the Elvish Archer / Orcish Grunt
combo, cherry-picked from gfgtdf's PR #7589. This is a unit with a 10x1
melee attack, and 0% defense on all terrains.
Remove the test race's undead_variation=
For testing custom plague abilities, this triggered a warning when
converting to any unit type that doesn't have variation_id=wolf,
restricting tests to using only Walking Corpse and Soulless.
