VkTestUtils: respect new optional SK_GPU_TOOLS_VK_LIBRARY_NAME define

Motivation:  someone who wants to build against swiftshader might want
to set to "libvk_swiftshader.so".

args.gn:
    extra_cflags=["-DSK_GPU_TOOLS_VK_LIBRARY_NAME=\"libvk_swiftshader.so\""]

Change-Id: I33174d58aa047739020c27996ce14254755b0c72
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215420
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
Cherry-Pick: 48cd11f004e17c6bbdbf16b2f82dcd359c101724
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215452
1 file changed