Create separate FVT workflow to test `--namespace-scope-mode`

[ckadner]

Issue #433 revealed that our current FVT setup is insufficient. We should create a separate FVT workflow to test the --namespace-scope-mode. The namespace-scope mode is used for the Quickstart installation

./scripts/install.sh --namespace-scope-mode --namespace modelmesh-serving --quickstart --enable-self-signed-ca

The new workflow should run in parallel to the existing cluster-scope FVT to not increase the overall time for PR checks to finish.

Originally posted by @ckadner in #433 (comment)