rewrite new SkVx unit test

 - be more explicit about casting
 - general rewrite for clarity

Cq-Include-Trybots: skia.primary:Test-Win2016-MSVC-GCE-CPU-AVX2-x86_64-Debug-All-MSRTC
Change-Id: I924d6d247e6b9afcefb27c690715fdad84635a5d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207721
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed