MultiviewRenderTestBase is renamed to MultiviewFramebufferTestBase
since the functionality it has is mostly to set up a framebuffer.

This cleans up the test class without including functional changes to
ease diagnosis of issues with resource init/teardown.

BUG=angleproject:2765
TEST=angle_end2end_tests

Change-Id: I0debd6a7a53a2e6bff7100d07b29313d66a79cb5
Reviewed-on: https://chromium-review.googlesource.com/1181052Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>