Updated test that was causing composer error

Lukas von Blarerrequested to merge
issue/views_infinite_scroll-2950901:2950901-custom-threshold-delay into 2.0.x

The InfiniteScrollTest.php file was trying to replace protected $defaultTheme = 'classy'; which is no longer present. Updating it to protected $defaultTheme = 'starterkit_theme'; to match the 2.0.x dev branch installs the patch successfully.

Closes #2950901

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