Unskip ShaderStorageBufferTest tests that reference bug angleproject:2990,
the bug is marked fixed and the tests pass.

Bug: angleproject:2990
Change-Id: I378f648baa1caf44c5e5280c1580abaefcbe6cba
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1742146
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>