Add base class for GLTestContext and add new subclass VkTestContext.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1964243003

Review-Url: https://codereview.chromium.org/1964243003
9 files changed