Use arena slugs in arena sign update handler.

The arena signs predate the use of arena slugs everywhere, so something
slipped through the cracks in this regard. Incidentally, the handler for
arena updates is one of the few classes in the signs package that has no
unit tests, probably due to it being "obvious implementation". Not so
obvious after all, it seems, so now we have a basic test for it.

Fixes #705

src/test/java/com/garbagemule/MobArena/signs/HandlesArenaUpdatesTest.java

@@ -0,0 +1,60 @@
+package com.garbagemule.MobArena.signs;
+
+import com.garbagemule.MobArena.MobArena;
+import com.garbagemule.MobArena.events.ArenaPlayerJoinEvent;
+import com.garbagemule.MobArena.framework.Arena;
+import org.bukkit.Server;
+import org.bukkit.scheduler.BukkitScheduler;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.junit.MockitoJUnitRunner;
+
+import java.util.Collections;
+
+import static org.mockito.Mockito.*;
+
+@SuppressWarnings("WeakerAccess")
+@RunWith(MockitoJUnitRunner.StrictStubs.class)
+public class HandlesArenaUpdatesTest {
+
+    SignStore signStore;
+    SignRenderer signRenderer;
+    BukkitScheduler scheduler;
+
+    HandlesArenaUpdates subject;
+
+    @Before
+    public void setup() {
+        signStore = mock(SignStore.class);
+        signRenderer = mock(SignRenderer.class);
+        scheduler = mock(BukkitScheduler.class);
+
+        MobArena plugin = mock(MobArena.class);
+        Server server = mock(Server.class);
+        when(plugin.getServer()).thenReturn(server);
+        when(server.getScheduler()).thenReturn(scheduler);
+
+        subject = new HandlesArenaUpdates(signStore, signRenderer, plugin);
+    }
+
+    @Test
+    public void usesArenaSlugForLookups() {
+        String slug = "angry-dingo";
+        Arena arena = mock(Arena.class);
+        ArenaSign sign = new ArenaSign(null, "cool-sign", slug, "info");
+        when(scheduler.runTask(any(), any(Runnable.class))).thenAnswer(i -> {
+            Runnable task = i.getArgument(1);
+            task.run();
+            return null;
+        });
+        when(arena.getSlug()).thenReturn(slug);
+        when(signStore.findByArenaId(slug)).thenReturn(Collections.singletonList(sign));
+        ArenaPlayerJoinEvent event = new ArenaPlayerJoinEvent(null, arena);
+
+        subject.on(event);
+
+        verify(signRenderer).render(sign);
+    }
+
+}