test: CmdAddWithResult sets CNI_PATH from PATH

To actually use CNI plugins in the given CNI_PATH, we need to add
CNI_PATH to PATH because CmdAddWithResult() does this:

os.Setenv("CNI_PATH", os.Getenv("PATH"))

test

@@ -48,7 +48,7 @@ fi
 split=(${TEST// / })
 TEST=${split[@]/#/${REPO_PATH}/}
 
-sudo -E bash -c "umask 0; PATH=${GOROOT}/bin:$(pwd)/bin:${PATH} CNI_PATH=${CNI_PATH} go test ${TEST}"
+sudo -E bash -c "umask 0; PATH=${GOROOT}/bin:$(pwd)/bin:${CNI_PATH}:${PATH} go test ${TEST}"
 
 echo "Checking gofmt..."
 fmtRes=$(gofmt -l $FMT)