Texture2DTest.TexStorageWithPBO/ES2_OpenGL is flaky.

Bug: b/168744561
Bug: angleproject:5097
Change-Id: Ide9b5a7d22e86c6eccd83ebc3bce6797b90150a6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2430163
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>