Use ManagedBackendTexture in place of TestUtils backend texture helpers.

Unify on a single family of helpers for making backend textures.

Stresses callback systems in more tests by avoiding artificial
flush/submit/syncs.

some misc test cleanup along the way.

Change-Id: Ia692e8927ba97b391ee77ea06ebf437a555617b2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/324710
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
24 files changed