Sector -2 0 fix (#322)

* Added a battle mission to Sector (-2,0) as otherwise no core would be spawned

* Fixed a crash which occurred during test execution...

... and could maybe occur during any startup with bad timing

* Added a new class for sector tests ...

... and added a test which makes sure that any predefined sector defines
a spawn point

* Added fix after applying test driven bugfixing

tests/src/test/java/SectorTests.java

@@ -0,0 +1,54 @@
+import com.badlogic.gdx.utils.Array;
+import io.anuke.mindustry.Vars;
+import io.anuke.mindustry.core.ContentLoader;
+import io.anuke.mindustry.maps.SectorPresets;
+import io.anuke.mindustry.maps.generation.Generation;
+import io.anuke.mindustry.maps.missions.Mission;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/** This class is responsible for testing predefined sectors. */
+public class SectorTests{
+
+    private SectorPresets presets;
+    private Generation fakeGen;
+
+    @BeforeAll
+    static void initializeDependencies(){
+        Vars.content = new ContentLoader();
+        Vars.content.load();
+    }
+
+    @BeforeEach
+    void initTest(){
+        this.presets = new SectorPresets();
+
+        // Fake away the Generation dependency
+        this.fakeGen = new Generation(null, null, 250, 250, null);
+    }
+
+    /** Returns true if at least one mission provides a spawn point. */
+    private boolean spawnPointIsDefined(Array<Mission> missions){
+        for(Mission mission : missions){
+            if(mission.getSpawnPoints(this.fakeGen).size > 0){
+                return true;
+            }
+        }
+        // No spawn point provided
+        return false;
+    }
+
+    /**
+     * Makes sure that every predefined sector has a position for the player core defined.
+     * This is achieved by adding at least one mission which defines a spawn point.
+     */
+    @Test
+    void test_sectorHasACore(){
+        for(SectorPresets.SectorPreset preset : this.presets.getPresets().values()){
+            assertTrue(spawnPointIsDefined(preset.missions), "Sector at (" + preset.x + "|" + preset.y + ") contains no missions which define a spawn point. Add a battle or wave mission.");
+        }
+    }
+}