Auto-scale related, Windows-specific tests are currently organized in a
class hierarchy based on Win32AutoscaleTestBase. This leads to
unnecessarily initialized objects (e.g., shells or displays without any
need for a test) and the strict necessity to use activated
monitor-specific scaling in actual tests or to revert the setup logic
affecting that value by hand.

This change removes the Win32AutoscaleTestBase and moves the common test
functionality into extensions (PlatformSpecificExecution,
ResetMonitorSpecificScaling and WithMonitorSpecificScaling) as well as
the utility class DPITestUtil.