Migrates a batch of tests from Mockito -> MockK

[jamesonwilliams]

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• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• MacBook Pro (unit test suite run)

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Description

Continuing to work towards removing Mockito.

This PR updates a batch of tests, migrating off of Mockito and towards MockK. I just did a grep for import.*mockito and grabbed a handful to update:

1. app/src/test/java/org/thoughtcrime/securesms/sms/UploadDependencyGraphTest.kt
2. app/src/test/java/org/thoughtcrime/securesms/stories/StoriesTest.kt
3. app/src/test/java/org/thoughtcrime/securesms/stories/StoryFirstTimeNavigationViewTest.kt
4. app/src/test/java/org/thoughtcrime/securesms/stories/dialogs/StoryContextMenuTest.kt
5. app/src/test/java/org/thoughtcrime/securesms/stories/viewer/StoryViewerViewModelTest.kt
6. app/src/test/java/org/thoughtcrime/securesms/stories/viewer/page/StoryViewerPageViewModelTest.kt
7. app/src/test/java/org/thoughtcrime/securesms/util/BackupUtilTest.kt
8. app/src/test/java/org/thoughtcrime/securesms/video/exo/ExoPlayerPoolTest.kt

After this, there are 10 more tests using Mockito. I'll put up a separate PR to migrate those ones.

Testing

This still passes:

./gradlew qa