[Impeller] flutter_tester --enable-impeller (#46389)

This patch does the following:

- Updates `flutter_tester` to set up an Impeller rendering context and surface if `--enable-impeller` is set to true, using the Vulkan backend with Swiftshader.
- Updates `run_tests.py` to run all tests except the smoke test (that one really has no rendering impact whatsoever) with and without `--enable-impeller`.
- Updates a few tests to work that were trivial:
  - A couple tests needed updated goldens for very minor rendering differences. Filed https://github.com/flutter/flutter/issues/135684 to track using Skia gold for this instead.
  - Disabled SKP screenshotting if Impeller is enabled, and updated the test checking that to verify an error is thrown if an SKP is requested.
  - The Dart GPU based test now asserts that the gpu context is available if Impeller is enabled, and does not deadlock if run in a single threaded mode.
  - We were missing some trace events around `Canvas::SaveLayer` for Impeller as compared to Skia.
  - A couple other tests had strict checks about exception messages that are slightly different between Skia and Impeller.
- I've filed bugs for other tests that may require a little more work, and skipped them for now. For FragmentProgram on Vulkan I reused an existing bug.

This is part of my attempt to address https://github.com/flutter/flutter/issues/135693, although @chinmaygarde and I had slightly different ideas about how to do this.

The goals here are:

- Run the Dart unit tests we already have with Impeller enabled.
- Enable running more of the framework tests (including gold tests) with Impeller enabled.
- Run all of these tests via public `dart:ui` API rather than mucking around in C++ internals in the engine.