ash_unittests should exercise production display management codepaths

Chrome OS uses DisplayConfigurator for on-device (production) display management/config.
But ash_unittests generally don't use this component, even when the focus of the test is display management/config.

1) AshTestBase automatically appends a default kHostWindowBounds commandline switch via AshTestHelper::SetUp:
   https://cs.chromium.org/chromium/src/ash/test/ash_test_helper.cc?rcl=4667c1c87d4746c53e39be01bd6b7bc4dd8e75bf&l=84
2) That causes ash::Shell::Init to deviate from the on-device production display management/config codepath:
   https://cs.chromium.org/chromium/src/ash/shell.cc?rcl=4667c1c87d4746c53e39be01bd6b7bc4dd8e75bf&l=886

So ash_unittests fakes everything out at the display::DisplayManager level and never test DisplayConfigurator.
There are some specific display_unittests exercising DisplayConfigurator, but the pattern above seems bad.
It seems like ash_unittests should generally use DisplayConfigurator, at least going forward in mus/mash.

Maybe linux-desktop Chrome OS should also use DisplayConfigurator in cash (it already does in mus/mash).

Please weigh in with any thoughts or advice, thanks! (I'm still not very familiar with this area)