rename sk_gpu_test::GLContext to sk_gpu_test::GLTestContext

rename subclasses

Fix up the EGL native GLTestContext
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1849463002

TBR=jvanverth@google.com

Committed: https://skia.googlesource.com/skia/+/4c7f0a16312c374eba4e8d5d46435ce9eb0b9971

Review URL: https://codereview.chromium.org/1849463002
34 files changed