test: add missing linkx test

This test was added in CL 151000043.
It got lost in CL 144630044.

Change-Id: I318ab11be8e3e7489fc1395457c029c8bdb2aa41
Reviewed-on: https://go-review.googlesource.com/1773Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/linkx_run.go

@@ -16,7 +16,7 @@ import (
 )
 
 func main() {
-	cmd := exec.Command("go", "run", "-ldflags=-X main.tbd hello -X main.overwrite trumped", "linkx.go")
+	cmd := exec.Command("go", "run", "-ldflags=-X main.tbd hello -X main.overwrite trumped -X main.nosuchsymbol neverseen", "linkx.go")
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		fmt.Println(string(out))