improve BookmarksViewModelTest coverage

[hiroaki404]

What I have done and why

• Improve coverage of BookmarksViewModel
• Add test for setNewsResourceViewed method and undoBookmarkRemoval method

Fixes #1333

How I'm testing it

• Unit tests

Do tests pass?

• Run local tests on DemoDebug variant: ./gradlew testDemoDebug
• Check formatting: ./gradlew --init-script gradle/init.gradle.kts spotlessApply

Is this your first pull request?

• Sign the CLA
• Run ./tools/setup.sh
• Import the code formatting style as explained in the setup script.

[google-cla]

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