[scenic] Add simple pixel tests
Adds two tests:
* Renders a solid color. All pixels are asserted to be this color.
* Renders a static square. Screenshot is saved to a file for consumption by
Skia Gold (integration to come later; this does not yet do a diff).
This test only executes on hardware (see changes to bin/ui/BUILD.gn).
Test: fx run-test scenic_tests -t gfx_pixeltests
Replaced solid color with square to verify failure case.
Change-Id: I58b8f77d14c60be9a69cfc1141cd2e0e4603a727
4 files changed
