wgengine/magicsock: set a dummy private key in benchmark.

Magicsock started dropping all traffic internally when Tailscale is
shut down, to avoid spurious wireguard logspam. This made the benchmark
not receive anything. Setting a dummy private key is sufficient to get
magicsock to pass traffic for benchmarking purposes.

Fixes #1270.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -1440,6 +1440,7 @@ func BenchmarkReceiveFrom(b *testing.B) {
 			},
 		},
 	})
+	conn.SetPrivateKey(wgkey.Private{0: 1})
 	conn.CreateEndpoint([32]byte{1: 1}, "0000000000000000000000000000000000000000000000000000000000000001.disco.tailscale:12345")
 	conn.addValidDiscoPathForTest(discoKey, netaddr.MustParseIPPort(sendConn.LocalAddr().String()))