Use ManagedBackendTexture in backend allocation tests

Change-Id: I96e598c989bea7d4ee08e12369fe1a5d83453ec3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/344759
Commit-Queue: Brian Salomon <[email protected]>
Reviewed-by: Robert Phillips <[email protected]>

Author: bsalomon