Add [test_do_attack_by_id], a wrapper for [do_command][attack]

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.

data/test/_main.cfg

@@ -29,6 +29,10 @@
 #Load test macros
 {test/macros}
 
+[lua]
+    code = "wesnoth.dofile 'test/lua/wml_tags.lua'"
+[/lua]
+
 # Load automated test scenarios.
 #
 # They can be run individually with Wesnoth's `-u` command line option, but are usually run by

data/test/lua/wml_tags.lua

@@ -0,0 +1,48 @@
+--! #textdomain wesnoth-test
+
+local T = wml.tag
+
+---Wrapper for [do_command][attack] that takes unit ids instead of map coordinates,
+---as tests are likely to already know the id.
+---
+---cfg.attacker,cfg.defender are unit ids, and map to [attack]source,destination=
+---cfg.weapon,cfg.defender_weapon are optional, and map to [attack]weapon,defender_weapon=
+---
+---cfg.resupply_attacks_left is an optional int, if set then it will ensure the attacker has
+---at least that many attacks_left before calling [do_command][attack].
+function wesnoth.wml_actions.test_do_attack_by_id(cfg)
+	-- can't be called "source", otherwise wmllint wants to add translation marks
+	if not cfg.attacker then
+		wml.error("[test_do_attack] missing required attacker= attribute")
+	end
+	local attacker = wesnoth.units.find_on_map { id = cfg.attacker }[1]
+	if not attacker or not attacker.valid then
+		wml.error("[test_do_attack] attacker did not match a unit")
+	end
+
+	if not cfg.defender then
+		wml.error("[test_do_attack] missing required defender= attribute")
+	end
+	local defender = wesnoth.units.find_on_map { id = cfg.defender }[1]
+	if not defender or not defender.valid then
+		wml.error("[test_do_attack] defender did not match a unit")
+	end
+
+	local weapon = cfg.weapon or ""
+	local defender_weapon = cfg.defender_weapon or ""
+
+	-- Avoid needing to modify units to give them multiple attacks per round
+	-- This attribute is a number rather than a boolean, to support [attack]attacks_used=
+	if cfg.resupply_attacks_left and attacker.attacks_left < cfg.resupply_attacks_left then
+		attacker.attacks_left = cfg.resupply_attacks_left
+	end
+
+	wesnoth.wml_actions.do_command {
+		T.attack {
+			T.source { x = attacker.x, y = attacker.y },
+			T.destination { x = defender.x, y = defender.y },
+			weapon = weapon,
+			defender_weapon = defender_weapon,
+		}
+	}
+end

data/test/units.cfg

@@ -10,7 +10,6 @@
         plural_name= _ "tests"
         description= _ "test race"
         num_traits=2
-        undead_variation=wolf
         {ORCISH_NAMES}
     [/race]
 
@@ -32,4 +31,48 @@
             note={INTERNAL:SPECIAL_NOTES_DEFENSE_CAP}
         [/special_note]
     [/movetype]
+
+    # The main feature of this movetype is that it has 0% defense on all terrains,
+    # so there's no need to use CHANCE_TO_HIT macros.
+    [movetype]
+        name=test_movetype_no_defense
+        [movement_costs]
+            shallow_water=2
+            reef=2
+            swamp_water=2
+            flat=2
+            sand=2
+            forest=2
+            hills=2
+            village=2
+            castle=2
+            cave=2
+            frozen=2
+            fungus=2
+        [/movement_costs]
+
+        [defense]
+            shallow_water=100
+            reef=100
+            swamp_water=100
+            flat=100
+            sand=100
+            forest=100
+            hills=100
+            village=100
+            castle=100
+            cave=100
+            frozen=100
+            fungus=100
+        [/defense]
+
+        [resistance]
+            blade=100
+            pierce=100
+            impact=100
+            fire=100
+            cold=100
+            arcane=100
+        [/resistance]
+    [/movetype]
 [/units]

data/test/units/test_melee_quintain.cfg

@@ -0,0 +1,34 @@
+#textdomain wesnoth-test
+
+# A Unit with simple stats for testing attack & ability releated functions.
+#
+# Neutral, and a single melee attack that only has one strike.
+[unit_type]
+    id=Test Melee Quintain
+    name=_ "test_unit^Test Melee Quintain"
+    race=test
+    # Image is a sword, to fit the melee-blade attack type. The icon is a non-flaming
+    # sword, because "-bare" is the version before the flames are added.
+    image="items/flame-sword-bare.png"
+    ignore_race_traits=yes
+    hitpoints=100
+    movement_type=test_movetype_no_defense
+    movement=20
+    experience=50
+    level=1
+    alignment=neutral
+    advances_to=null
+    cost=1
+    hide_help=yes
+    do_not_list=yes
+    num_traits=0
+    [attack]
+        name="melee_attack"
+        description=_"Melee Attack"
+        icon=attacks/sword-human.png
+        type=blade
+        range=melee
+        damage=10
+        number=1
+    [/attack]
+[/unit_type]