Add Dot.sksl to SkSLTest.cpp.

Looks like we have several intrinsics where the tests were written
but never updated to be runnable. Here's another.

Change-Id: Ic325b5bc23cffca76f8d7cd84f433a6c69866df1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/626199
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed