Add DatabaseMocker to mock NewPipeDatabase

app/src/androidTest/java/org/schabi/newpipe/testUtil/TestDatabase.kt

@@ -0,0 +1,32 @@
+package org.schabi.newpipe.testUtil
+
+import androidx.room.Room
+import androidx.test.core.app.ApplicationProvider
+import org.junit.Assert.assertSame
+import org.schabi.newpipe.NewPipeDatabase
+import org.schabi.newpipe.database.AppDatabase
+
+class TestDatabase {
+    companion object {
+        fun createReplacingNewPipeDatabase(): AppDatabase {
+            val database = Room.inMemoryDatabaseBuilder(
+                ApplicationProvider.getApplicationContext(),
+                AppDatabase::class.java
+            )
+                .allowMainThreadQueries()
+                .build()
+
+            val databaseField = NewPipeDatabase::class.java.getDeclaredField("databaseInstance")
+            databaseField.isAccessible = true
+            databaseField.set(NewPipeDatabase::class, database)
+
+            assertSame(
+                "Mocking database failed!",
+                database,
+                NewPipeDatabase.getInstance(ApplicationProvider.getApplicationContext())
+            )
+
+            return database
+        }
+    }
+}