Split VulkanTestHelper out into its own files

The aim here is to unify VkTestHelper with the VkYcbcrSamplerHelper's context creation code. AFAICT they have a ~90% overlap.

Change-Id: Iba8d1482b8c1d7164682f90e19d2183d8cfe45d4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295583
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 99f02aa..f6cbdb4 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1430,6 +1430,8 @@
     if (skia_use_vulkan) {
       sources += [ "tools/gpu/vk/VkTestContext.h" ]
       sources += [ "tools/gpu/vk/VkTestContext.cpp" ]
+      sources += [ "tools/gpu/vk/VkTestHelper.h" ]
+      sources += [ "tools/gpu/vk/VkTestHelper.cpp" ]
       sources += [ "tools/gpu/vk/VkTestUtils.h" ]
       sources += [ "tools/gpu/vk/VkTestUtils.cpp" ]
       sources += [ "tools/gpu/vk/VkYcbcrSamplerHelper.h" ]