Fix flaky Windows exit unit test, remove error messages (#40945)

Shutdown is async, so we can not guarantee the order of execution and,
by extension, ignorance of execution used as an auxiliary part of a unit
test. Also, mock the windows view/binding handler for the engine in
order to avoid error messages.

Addresses https://github.com/flutter/flutter/issues/124162

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tree: 8acc43344eb9e6e7d77d946065bdfcd21163b002
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  3. benchmarking/
  4. build/
  5. ci/
  6. common/
  7. display_list/
  8. docs/
  9. examples/
  10. flow/
  11. flutter_frontend_server/
  12. flutter_vma/
  13. fml/
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  15. lib/
  16. runtime/
  17. shell/
  18. sky/
  19. testing/
  20. third_party/
  21. tools/
  22. vulkan/
  23. wasm/
  24. web_sdk/
  25. .ci.yaml
  26. .clang-format
  27. .clang-tidy
  28. .gitattributes
  29. .gitignore
  30. .pylintrc
  31. .style.yapf
  32. analysis_options.yaml
  33. AUTHORS
  34. BUILD.gn
  35. CODEOWNERS
  36. CONTRIBUTING.md
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  40. README.md
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