tools/cmd/vet: check that cgo is enabled before testing it

Should fix the tools builders.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/176530043

cmd/stringer/endtoend_test.go

@@ -6,6 +6,7 @@ package main
 
 import (
 	"fmt"
+	"go/build"
 	"io"
 	"io/ioutil"
 	"os"
@@ -45,6 +46,10 @@ func TestEndToEnd(t *testing.T) {
 			t.Errorf("%s is not a Go file", name)
 			continue
 		}
+		if name == "cgo.go" && !build.Default.CgoEnabled {
+			t.Logf("cgo is no enabled for %s", name)
+			continue
+		}
 		// Names are known to be ASCII and long enough.
 		typeName := fmt.Sprintf("%c%s", name[0]+'A'-'a', name[1:len(name)-len(".go")])
 		stringerCompileAndRun(t, dir, stringer, typeName, name)