This is how we can apply the new test runner to remove duplication.

This commit was generated programmatically with the following commands:
```
  // Replace test runners with new one.
  find app/src/test -type f -exec gsed -i "s/@RunWith(RobolectricTestRunner::class)/@RunWith(FenixRobolectricTestRunner::class)/" {} +
  find app/src/test -type f -exec gsed -i "s/@RunWith(AndroidJUnit4::class)/@RunWith(FenixRobolectricTestRunner::class)/" {} +

  // Replace imports of old test runners with new one
  find app/src/test -type f -exec gsed -i "s/org.robolectric.RobolectricTestRunner/org.mozilla.fenix.helpers.FenixRobolectricTestRunner/" {} +
  find app/src/test -type f -exec gsed -i "s/androidx.test.ext.junit.runners.AndroidJUnit4/org.mozilla.fenix.helpers.FenixRobolectricTestRunner/" {} +

  // Remove unused imports
  find app/src/test -type f -exec gsed -i "/@Config(application = TestApplication::class)/d" {} +
  find app/src/test -type f -exec gsed -i "/import org.mozilla.fenix.TestApplication/d" {} +
  find app/src/test -type f -exec gsed -i "/import org.robolectric.annotation.Config/d" {} +
```

Where gsed is the GNU version of sed installed via homebrew. After
running these commands, I need to manually clean up the following files:
- FenixRobolectricTestRunner
- LocaleManagerExtensionTest