vet: ensure all usages of grpc_testing package are renamed when impor…

…ting (#6595)

vet.sh

@@ -93,6 +93,9 @@ git grep -l -e 'grpclog.I' --or -e 'grpclog.W' --or -e 'grpclog.E' --or -e 'grpc
 # - Ensure all ptypes proto packages are renamed when importing.
 not git grep "\(import \|^\s*\)\"github.com/golang/protobuf/ptypes/" -- "*.go"
 
+# - Ensure all usages of grpc_testing package are renamed when importing.
+not git grep "\(import \|^\s*\)\"google.golang.org/grpc/interop/grpc_testing" -- "*.go" 
+
 # - Ensure all xds proto imports are renamed to *pb or *grpc.
 git grep '"github.com/envoyproxy/go-control-plane/envoy' -- '*.go' ':(exclude)*.pb.go' | not grep -v 'pb "\|grpc "'