net/http/fcgi: fix test

Currently the test fails if run more than once:

$ go test -v -run=TestChildServeCleansUp -cpu=1,1 net/http/fcgi
=== RUN TestChildServeCleansUp
--- PASS: TestChildServeCleansUp (0.00s)
=== RUN TestChildServeCleansUp
fatal error: all goroutines are asleep - deadlock!

The problem is that the writer mutates test input data,
so it is wrong on the second execution.

Change-Id: I4ca54dd2926c6986b2908023ac65e5e65630ed26
Reviewed-on: https://go-review.googlesource.com/6383Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/fcgi/fcgi_test.go

@@ -233,7 +233,9 @@ func (nopWriteCloser) Close() error {
 // isn't met. See issue 6934.
 func TestChildServeCleansUp(t *testing.T) {
 	for _, tt := range cleanUpTests {
-		rc := nopWriteCloser{bytes.NewBuffer(tt.input)}
+		input := make([]byte, len(tt.input))
+		copy(input, tt.input)
+		rc := nopWriteCloser{bytes.NewBuffer(input)}
 		done := make(chan bool)
 		c := newChild(rc, http.HandlerFunc(func(
 			w http.ResponseWriter,