net: disable TestDialMultiFDLeak

It fails about 25% of the time on OS X.
I don't know what it's trying to do.
Created issue 8764 to correct this, but for now disable.

LGTM=bradfitz, mikioh.mikioh
R=bradfitz, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/144070044

src/net/dial_test.go

@@ -339,6 +339,8 @@ func numTCP() (ntcp, nopen, nclose int, err error) {
 }
 
 func TestDialMultiFDLeak(t *testing.T) {
+	t.Skip("flaky test - golang.org/issue/8764")
+
 	if !supportsIPv4 || !supportsIPv6 {
 		t.Skip("neither ipv4 nor ipv6 is supported")
 	}