* add comment to skipped safari test

* test not running semantics on Mac

* disable semantics test which is unreliable

Also enable tests that were disabled due to MessageLoop flakiness resolved by https://github.com/flutter/engine/pull/16568

* Safari font collection tests skip failing methods

* add a clear to the semantics test to make sure no DOM elements are leaking to the other tests

* fix semantics_helper_test. skip for semantics_test. teardown didn't work

* skip test, the skipped method run for 8 minutes according to the logs

* history tests methods runs multiple times. only run them on linux for now. they were also already failing in edge/windows

Flush the SkCanvas when submitting a frame in ShellTestPlatformViewVulkan::OffscreenSurface (#16717)

This change creates a test only implementation of flutter::Surface backed by an
offscreen Vulkan GrContext. Much of the code in this test Surface was lifted
from flutter::VulkanWindow which I was unable to use without extricating it
from the VkSurface/VkSwapchain code which we do not want to use in offscreen
tests. I would recommend refactoring VulkanWindow to separate GrContext
creation and VkSwapchain creation in order to promote greater code reuse
between onscreen and offscreen vulkan paths.

This change is excersised thoroughly by the shell tests and was manually
tested against these tests on Fuchsia on Intel.

* opt out dart:ui from nnbd

* Annotate tests as well