ComputerBooter is now an abstract class, so tests that tried to
instantiate it directly need to be updated:
- test_booter_decoupling.py: remove test_get_tools_delegates_to_class
since base class cannot be instantiated
- test_profile_aware_tools.py: use ShipyardBooter.__new__() to test
base class property defaults (capabilities, browser)
- test_computer.py: skip BoxliteBooter test since it's also abstract
and requires the boxlite module