cmd/cgo/internal/testfortran: relax test output

Some new linker may emit warning message to standard error, causing
false positive in test result.

Fixing this by testing only stdout output.

Fixes #63588

Change-Id: I272048c41dc1c316f44af2dfc903bb03383baea3
Reviewed-on: https://go-review.googlesource.com/c/go/+/535975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/cgo/internal/testfortran/fortran_test.go

@@ -5,7 +5,6 @@
 package fortran
 
 import (
-	"fmt"
 	"internal/testenv"
 	"os"
 	"os/exec"
@@ -75,11 +74,18 @@ func TestFortran(t *testing.T) {
 
 	// Finally, run the actual test.
 	t.Log("go", "run", "./testdata/testprog")
-	out, err := exec.Command("go", "run", "./testdata/testprog").CombinedOutput()
-	if err == nil && string(out) != "ok\n" {
-		err = fmt.Errorf("expected ok")
+	var stdout, stderr strings.Builder
+	cmd := exec.Command("go", "run", "./testdata/testprog")
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	err := cmd.Run()
+	t.Logf("%v", cmd)
+	if stderr.Len() != 0 {
+		t.Logf("stderr:\n%s", stderr.String())
 	}
 	if err != nil {
-		t.Errorf("%s\nOutput:\n%s", err, string(out))
+		t.Errorf("%v\n%s", err, stdout.String())
+	} else if stdout.String() != "ok\n" {
+		t.Errorf("stdout:\n%s\nwant \"ok\"", stdout.String())
 	}
 }

src/cmd/cgo/internal/testfortran/testdata/testprog/fortran.go

@@ -6,7 +6,10 @@ package main
 
 // int the_answer();
 import "C"
-import "os"
+import (
+	"fmt"
+	"os"
+)
 
 func TheAnswer() int {
 	return int(C.the_answer())
@@ -14,8 +17,8 @@ func TheAnswer() int {
 
 func main() {
 	if a := TheAnswer(); a != 42 {
-		println("Unexpected result for The Answer. Got:", a, " Want: 42")
+		fmt.Fprintln(os.Stderr, "Unexpected result for The Answer. Got:", a, " Want: 42")
 		os.Exit(1)
 	}
-	println("ok")
+	fmt.Fprintln(os.Stdout, "ok")
 }