[graphite] Move testing ContextOptions to private struct

Bug: b/294389814
Change-Id: I2f902839318c0f4d0dda9d497a883723b6a0727b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/755552
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
14 files changed