Add test for DDLs and SkSurface::flush

Bug: 958055
Change-Id: I9abffd09344a731efbd29dce23db76d56d258622
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212408
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed