Use an in-memory database when running instrumented tests.

- added new TestRunner and new TestApplication
- The TestRunner loads TestApplication as the apps Application instance when running the tests
- added new interface AppConfig to retrieve the database name
- changed Startup (Application) so that it implements AppConfig
- changed CustomSQLiteOpenHelper so that it retrieves the database name from the Application instance
- Startup provides the production database name
- TestApplication provides null which yields an in-mememory database

src/androidTest/java/de/dennisguse/opentracks/TestApplication.java

@@ -0,0 +1,9 @@
+package de.dennisguse.opentracks;
+
+public class TestApplication extends Startup {
+    @Override
+    public String getDatabaseName() {
+        // null will make SQLiteOpenHelper create an in-memory database
+        return null;
+    }
+}