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/TestRunner.java

@@ -0,0 +1,14 @@
+package de.dennisguse.opentracks;
+
+import android.app.Application;
+import android.content.Context;
+
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import androidx.test.runner.AndroidJUnitRunner;
+
+public class TestRunner extends AndroidJUnitRunner {
+    @Override
+    public Application newApplication(ClassLoader cl, String className, Context context) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
+        return super.newApplication(cl, TestApplication.class.getName(), context);
+    }
+}