cmd/compile/internal/syntax: silence test function output

Don't print to stdout in non-verbose (-v) test mode.

Exception: Timing output (2 lines) of TestStdLib. If
we want to disable that as well we should use another
flag to differenciate between -verbose output and
measurement results. Leaving alone for now.

Fixes #35223.

Change-Id: Ie8160760e8db1138f9031888d654eaeab202128c
Reviewed-on: https://go-review.googlesource.com/c/go/+/204039
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/syntax/dumper_test.go

@@ -5,7 +5,6 @@
 package syntax
 
 import (
-	"os"
 	"testing"
 )
 
@@ -21,6 +20,6 @@ func TestDump(t *testing.T) {
 	}
 
 	if ast != nil {
-		Fdump(os.Stdout, ast)
+		Fdump(testOut(), ast)
 	}
 }

src/cmd/compile/internal/syntax/printer_test.go

@@ -6,6 +6,8 @@ package syntax
 
 import (
 	"fmt"
+	"io"
+	"io/ioutil"
 	"os"
 	"strings"
 	"testing"
@@ -23,7 +25,7 @@ func TestPrint(t *testing.T) {
 	}
 
 	if ast != nil {
-		Fprint(os.Stdout, ast, true)
+		Fprint(testOut(), ast, true)
 		fmt.Println()
 	}
 }
@@ -44,3 +46,10 @@ func TestPrintString(t *testing.T) {
 		}
 	}
 }
+
+func testOut() io.Writer {
+	if testing.Verbose() {
+		return os.Stdout
+	}
+	return ioutil.Discard
+}

src/cmd/compile/internal/syntax/scanner_test.go

@@ -30,6 +30,9 @@ func TestScanner(t *testing.T) {
 		if s.tok == _EOF {
 			break
 		}
+		if !testing.Verbose() {
+			continue
+		}
 		switch s.tok {
 		case _Name:
 			fmt.Println(s.line, s.tok, "=>", s.lit)