For #1710: Create scaffolding for Robolectric tests

[colintheshots]

I added a basic Robolectric integration. This required downgrading to JUnit 4. I had to mock out parts of the application like background sync, but couldn't do a proper test of the bookmarks fragment yet because the Android Component had its CoroutineScope set as internal visibility only.

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[boek]

Looks like a handful of builds are failing at:

e: /opt/fenix/app/src/test/java/org/mozilla/fenix/library/bookmarks/BookmarkFragmentTest.kt: (7, 30): Unresolved reference: testing
e: /opt/fenix/app/src/test/java/org/mozilla/fenix/library/bookmarks/BookmarkFragmentTest.kt: (8, 30): Unresolved reference: testing
e: /opt/fenix/app/src/test/java/org/mozilla/fenix/library/bookmarks/BookmarkFragmentTest.kt: (32, 36): Unresolved reference: FragmentScenario
e: /opt/fenix/app/src/test/java/org/mozilla/fenix/library/bookmarks/BookmarkFragmentTest.kt: (43, 13): Unresolved reference: launchFragmentInContainer
e: /opt/fenix/app/src/test/java/org/mozilla/fenix/library/bookmarks/BookmarkFragmentTest.kt: (56, 31): Cannot infer a type for this parameter. Please specify it explicitly.