[magma] vkloop - Move to vulkan context.

This change reduces test size and by moving to vulkan context it
has fewer vulkan memory leaks upon destruction of test class
instance.

Bug: 48555

Change-Id: Ib9bad8477fdbb02a59897794d16169d7cbdd7f6f
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/376755
Commit-Queue: John Rosasco <rosasco@google.com>
Reviewed-by: Craig Stout <cstout@google.com>
Testability-Review: Craig Stout <cstout@google.com>