fix Dark Impostor and add test coverage

Mage.Sets/src/mage/cards/d/DarkImpostor.java

@@ -102,7 +102,7 @@ class DarkImpostorContinuousEffect extends ContinuousEffectImpl {
     @Override
     public boolean apply(Game game, Ability source) {
         Permanent permanent = source.getSourcePermanentIfItStillExists(game);
-        ExileZone exileZone = game.getExile().getExileZone(CardUtil.getExileZoneId(game, source));
+        ExileZone exileZone = game.getExile().getExileZone(CardUtil.getExileZoneId(game, source, 1));
         if (permanent == null || exileZone == null || exileZone.isEmpty()) {
             return false;
         }

Mage.Tests/src/test/java/org/mage/test/cards/single/avr/DarkImpostorTest.java

@@ -0,0 +1,53 @@
+package org.mage.test.cards.single.avr;
+
+import mage.abilities.common.SimpleActivatedAbility;
+import mage.constants.PhaseStep;
+import mage.constants.Zone;
+import org.junit.Test;
+import org.mage.test.serverside.base.CardTestPlayerBase;
+
+public class DarkImpostorTest extends CardTestPlayerBase {
+
+    /*
+    Dark Impostor
+    {2}{B}
+    Creature — Vampire Assassin
+
+    {4}{B}{B}: Exile target creature and put a +1/+1 counter on this creature.
+
+    This creature has all activated abilities of all creature cards exiled with it.
+    2/2
+     */
+    public static final String darkImposter = "Dark Impostor";
+    /*
+    Deathrite Shaman
+    {B/G}
+    Creature — Elf Shaman
+
+    {T}: Exile target land card from a graveyard. Add one mana of any color.
+
+    {B}, {T}: Exile target instant or sorcery card from a graveyard. Each opponent loses 2 life.
+
+    {G}, {T}: Exile target creature card from a graveyard. You gain 2 life.
+
+    1/2
+     */
+    public static final String deathriteShaman = "Deathrite Shaman";
+
+    @Test
+    public void testDarkImpostor() {
+        setStrictChooseMode(true);
+
+        addCard(Zone.BATTLEFIELD, playerA, darkImposter);
+        addCard(Zone.BATTLEFIELD, playerA, deathriteShaman);
+        addCard(Zone.BATTLEFIELD, playerA, "Swamp", 6);
+
+        activateAbility(1, PhaseStep.PRECOMBAT_MAIN, playerA, "{4}{B}{B}");
+        addTarget(playerA, deathriteShaman);
+
+        setStopAt(1, PhaseStep.END_TURN);
+        execute();
+
+        assertAbilityCount(playerA, darkImposter, SimpleActivatedAbility.class, 4); // own ability + 3 other from deathrite
+    }
+}