[Impeller] disabled misleading vulkan golden image tests (flutter/engine#49836)

issue: https://github.com/flutter/flutter/issues/141705

The MetalScreenshotter is hardcoded to make a Metal context. So even
though the tests were running the Vulkan variant, the rendering was
still happening with Metal.

It's better to disable these for now until they can be implemented
properly (either by making them render with swiftshader on mac or
rendering them on linux).

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instructions on writing and running engine tests.
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engine/src/flutter/impeller/golden_tests/golden_playground_test_mac.cc

@@ -122,8 +122,7 @@ void GoldenPlaygroundTest::SetUp() {
   std::filesystem::path icd_path = target_path / "vk_swiftshader_icd.json";
   setenv("VK_ICD_FILENAMES", icd_path.c_str(), 1);
 
-  if (GetBackend() != PlaygroundBackend::kMetal &&
-      GetBackend() != PlaygroundBackend::kVulkan) {
+  if (GetBackend() != PlaygroundBackend::kMetal) {
     GTEST_SKIP_("GoldenPlaygroundTest doesn't support this backend type.");
     return;
   }