gitignore: cleanup stale entries (#37336)
gitignore: cleanup stale entries (#37336)

Aside from not being relevant anymore, if you happen to have these old
test files around (which are no longer generated), the tests will fail.
So make it obvious they exist and let `git clean` remove them.
istio/istio,istio/istio,istio/istio,istio/istio,istio/istio