Enable Swiftshader for Vulkan testing on Mac.

This allows dm to run basic Vulkan tests, albeit without GPU
acceleration.

Change-Id: Idb738125ad302054be290d50f92b494d6fc6a84c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/501686
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Christopher Dalton <csmartdalton@google.com>
Commit-Queue: Christopher Dalton <csmartdalton@google.com>
diff --git a/tools/gpu/vk/VkTestUtils.cpp b/tools/gpu/vk/VkTestUtils.cpp
index 5720cbe..9740393 100644
--- a/tools/gpu/vk/VkTestUtils.cpp
+++ b/tools/gpu/vk/VkTestUtils.cpp
@@ -12,6 +12,8 @@
 #ifndef SK_GPU_TOOLS_VK_LIBRARY_NAME
     #if defined _WIN32
         #define SK_GPU_TOOLS_VK_LIBRARY_NAME vulkan-1.dll
+    #elif defined SK_BUILD_FOR_MAC
+        #define SK_GPU_TOOLS_VK_LIBRARY_NAME libvk_swiftshader.dylib
     #else
         #define SK_GPU_TOOLS_VK_LIBRARY_NAME libvulkan.so
     #endif