http: adjust test threshold for larger suse buffers

My theory is that suse has larger TCP buffer sizes
by default.  I now check over 100MB, rather than over 2MB.
100MB is ~halfway between the 1MB limit and the 200MB
request that's attempted.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4926048

src/pkg/http/serve_test.go

@@ -944,7 +944,7 @@ func TestRequestBodyLimit(t *testing.T) {
 	// the remote side hung up on us before we wrote too much.
 	_, _ = DefaultClient.Do(req)
 
-	if nWritten > limit*2 {
+	if nWritten > limit*100 {
 		t.Errorf("handler restricted the request body to %d bytes, but client managed to write %d",
 			limit, nWritten)
 	}