test: return errors earlier in run.go

Fixes #8184.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/137510043

test/run.go

@@ -626,6 +626,7 @@ func (t *test) run() {
 		out, err := runcmd(append([]string{"go", "run", t.goFileName()}, args...)...)
 		if err != nil {
 			t.err = err
+			return
 		}
 		if strings.Replace(string(out), "\r\n", "\n", -1) != t.expectedOutput() {
 			t.err = fmt.Errorf("incorrect output\n%s", out)
@@ -640,6 +641,7 @@ func (t *test) run() {
 		out, err := runcmd(append([]string{"go", "run", t.goFileName()}, args...)...)
 		if err != nil {
 			t.err = err
+			return
 		}
 		tfile := filepath.Join(t.tempDir, "tmp__.go")
 		if err := ioutil.WriteFile(tfile, out, 0666); err != nil {
@@ -649,6 +651,7 @@ func (t *test) run() {
 		out, err = runcmd("go", "run", tfile)
 		if err != nil {
 			t.err = err
+			return
 		}
 		if string(out) != t.expectedOutput() {
 			t.err = fmt.Errorf("incorrect output\n%s", out)
@@ -659,6 +662,7 @@ func (t *test) run() {
 		out, err := runcmd(append([]string{"go", "run", t.goFileName()}, args...)...)
 		if err != nil {
 			t.err = err
+			return
 		}
 		tfile := filepath.Join(t.tempDir, "tmp__.go")
 		err = ioutil.WriteFile(tfile, out, 0666)