iOS: Eliminate use of IOSContextNoop in platform view tests (reland) - #190509

This relands #190419, which I reverted in #190501 because it broke the tree due to a (non-merge) conflict with #190416.

#190419 added a local FakeIOSContext whose GetBackend() override returned flutter::IOSRenderingBackend::kImpeller.

#190416 landed immediately before it and removed the IOSContext::GetBackend() virtual and the IOSRenderingBackend enum, so the two changes had no merge conflicts but FakeIOSContext no longer compiled once both were in the tree.

This reland is identical other than deleting the GetBackend() override. FakeIOSContext still inherits the null Impeller/Aiks contexts from the base class and returns no external texture, matching the behaviour the tests rely on.

Original change:

FlutterPlatformViewsTest used the internal engine class flutter::IOSContextNoop in a dozen places purely as a no-op context to hand to submitFrame:withIosContext:. This replaces those with a local FakeIOSContext that replicates the behaviour the tests rely on so that we can delete IOSContextNoop in a follow-up.

Back in ancient times, before the simulator required Metal, and when iOS still had a Skia software renderer, we ran that on the Simulator due to some issues with our OpenGL implementation. Later, Flutter on iOS migrated from Skia to Impeller. Skia supports a software backend but Impeller did not, and so when running on Impeller, we stubbed out the software backend to no-op context/surfaces, hence IOSContextNoop and IOSSurfaceNoop. The Simulator now requires Metal and Flutter has eliminated Skia support altogether so there's no longer a software mode at all, nor a need for a no-op path in case you're using Impeller and ask for a software backend.

This is part of a series of changes to remove the dead no-op fallback.

No behavioural change; just a test refactoring.

Issue: #190041
Issue: #112232

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