test: improve 'getLatestAlertDialog' error

Previously this resulted in a `NullPointerException`

before:
```
java.lang.NullPointerException: null cannot be cast to non-null type androidx.appcompat.app.AlertDialog
```

after:
```
org.opentest4j.AssertionFailedError: A dialog should be displayed ==> expected: not <null>
```

Seen in https://redirect.github.com/ankidroid/Anki-Android/pull/16529

AnkiDroid/src/test/java/com/ichi2/anki/RobolectricTest.kt

@@ -65,6 +65,7 @@ import org.robolectric.shadows.ShadowLog
 import org.robolectric.shadows.ShadowLooper
 import org.robolectric.shadows.ShadowMediaPlayer
 import timber.log.Timber
+import kotlin.test.assertNotNull
 
 open class RobolectricTest : AndroidTest {
 
@@ -197,7 +198,7 @@ open class RobolectricTest : AndroidTest {
      * Click on a dialog button for an AlertDialog dialog box. Replaces the above helper.
      */
     protected fun clickAlertDialogButton(button: Int, @Suppress("SameParameterValue") checkDismissed: Boolean) {
-        val dialog = ShadowDialog.getLatestDialog() as AlertDialog
+        val dialog = getLatestAlertDialog()
 
         dialog.getButton(button).performClick()
         // Need to run UI thread tasks to actually run the onClickHandler
@@ -216,7 +217,7 @@ open class RobolectricTest : AndroidTest {
      * TODO: Rename to getDialogText when all MaterialDialogs are changed to AlertDialogs
      */
     protected fun getAlertDialogText(@Suppress("SameParameterValue") checkDismissed: Boolean): String? {
-        val dialog = ShadowDialog.getLatestDialog() as AlertDialog
+        val dialog = getLatestAlertDialog()
         if (checkDismissed && Shadows.shadowOf(dialog).hasBeenDismissed()) {
             Timber.e("The latest dialog has already been dismissed.")
             return null
@@ -480,3 +481,6 @@ open class RobolectricTest : AndroidTest {
         }
     }
 }
+
+private fun getLatestAlertDialog(): AlertDialog =
+    assertNotNull(ShadowDialog.getLatestDialog() as? AlertDialog, "A dialog should be displayed")