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)
 	}
 }