Minor angle_end2end_tests cleanups. Minor refactoring and call consolidation. Was done while working on deferring clears in the Vulkan back-end. Bug: angleproject:4517 Change-Id: I60d80d4f9646c990c4cb8bbc349fc0051389bf02 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2151171 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>