mime/multipart: fix Writer data race test

If the process exits before the spawned goroutine
completes, it'll miss the data race.

LGTM=bradfitz
R=bradfitz
CC=dvyukov, golang-codereviews
https://golang.org/cl/122120043

src/pkg/mime/multipart/writer_test.go

@@ -118,8 +118,11 @@ func TestWriterBoundaryGoroutines(t *testing.T) {
 	// https://codereview.appspot.com/95760043/ and reverted in
 	// https://codereview.appspot.com/117600043/
 	w := NewWriter(ioutil.Discard)
+	done := make(chan int)
 	go func() {
 		w.CreateFormField("foo")
+		done <- 1
 	}()
 	w.Boundary()
+	<-done
 }