tests:Fix memory leak in MipmapTest
tests:Fix memory leak in MipmapTest

Update createRGBInitData() to use/return std::vector instead of raw ptr
w/ "new." This prevents potential for memory leaks.

Bug: angleproject:2958
Change-Id: I4b1bb4ee293e171f56d1221a562e607ff8eb0fb3
Reviewed-on: https://chromium-review.googlesource.com/c/1375229
Reviewed-by: Yuly Novikov <36f5580f63d0a3eacc17e89a61d11e2f646630d2@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Tobin Ehlis <c15ba1c6ec8841cd136fafb26b333b1f76c86fb4@google.com>

ppy/angle,ppy/angle,ppy/angle,ppy/angle