By doing the copy multiple times, we exercise both paths where the
destination is already initialized and when it's not.

This adds tests for all combinations of formats and flags.

Bug: angleproject:2958
Change-Id: I56afb44496acd1b4d5a8527f4dbee29afbac9c81
Reviewed-on: https://chromium-review.googlesource.com/c/1398643
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>