fix warnings in test runs (#4340)

Found while working on #4026 but not directly related.

Two cases of unmocked methods and one of unclosed resource.

app/src/test/java/com/keylesspalace/tusky/MainActivityTest.kt

@@ -18,6 +18,7 @@ import com.keylesspalace.tusky.entity.Notification
 import com.keylesspalace.tusky.entity.TimelineAccount
 import java.util.Date
 import kotlinx.coroutines.test.TestScope
+import org.junit.After
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertNotNull
 import org.junit.Before
@@ -60,6 +61,11 @@ class MainActivityTest {
         WorkManagerTestInitHelper.initializeTestWorkManager(context)
     }
 
+    @After
+    fun teardown() {
+        WorkManagerTestInitHelper.closeWorkDatabase()
+    }
+
     @Test
     fun `clicking notification of type FOLLOW shows notification tab`() {
         val intent = showNotification(Notification.Type.FOLLOW)