net: enable TestReadTimeoutUnblocksRead on Plan 9

Deadlines have been implemented on Plan 9 in CL 31521.

Fixes #17477.

Change-Id: Icb742ac30933b6d2f9350fc4e6acbcd433c66c21
Reviewed-on: https://go-review.googlesource.com/33190Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/net_test.go

@@ -457,9 +457,6 @@ func withTCPConnPair(t *testing.T, peer1, peer2 func(c *TCPConn) error) {
 // See golang.org/cl/30164 which documented this. The net/http package
 // depends on this.
 func TestReadTimeoutUnblocksRead(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		t.Skipf("not supported on %s; see golang.org/issue/17477", runtime.GOOS)
-	}
 	serverDone := make(chan struct{})
 	server := func(cs *TCPConn) error {
 		defer close(serverDone)