wgengine/filter: add test cases for len(dsts) > 1.

While the code was correct, I broke it during a refactoring and
tests didn't detect it. This fixes that glitch.

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

wgengine/filter/filter_test.go

@@ -54,6 +54,11 @@ func TestFilter(t *testing.T) {
 		{Accept, parsed(packet.TCP, "8.2.2.2", "1.2.3.4", 0, 22)},
 		{Drop, parsed(packet.TCP, "8.2.2.2", "1.2.3.4", 0, 23)},
 		{Drop, parsed(packet.TCP, "8.3.3.3", "1.2.3.4", 0, 22)},
+		// allow 8.1.1.1 => 5.6.7.8:23-24
+		{Accept, parsed(packet.TCP, "8.1.1.1", "5.6.7.8", 0, 23)},
+		{Accept, parsed(packet.TCP, "8.1.1.1", "5.6.7.8", 0, 24)},
+		{Drop, parsed(packet.TCP, "8.1.1.3", "5.6.7.8", 0, 24)},
+		{Drop, parsed(packet.TCP, "8.1.1.1", "5.6.7.8", 0, 22)},
 		// allow * => *:443
 		{Accept, parsed(packet.TCP, "17.34.51.68", "8.1.34.51", 0, 443)},
 		{Drop, parsed(packet.TCP, "17.34.51.68", "8.1.34.51", 0, 444)},