Specify supported test backends in CI (#306)
Specify supported test backends in CI (#306)

Vulkan is the default otherwise and it's not supported in the OSS CI

Tested:
I tried this out on my laptop, which doesn't have vulkan support and now I can run //integrations/tensorflow/e2e:exported_names_test
google/iree,google/iree,google/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree