MultithreadingTest.MultiContextClear/ES3_Vulkan_NoVirtual is flaky on
linux-clang-rel NVIDIA, so it's being suppressed for now.

Bug: angleproject:5945
Change-Id: I9ef142c57b3fcf5224732edcea1650634ecc18ca
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2887905
Commit-Queue: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>