r/aws_route: Add 'TestAccAWSRoute_IPv6_To_LocalGateway'.

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSRoute_IPv6_To_LocalGateway' ACCTEST_PARALLELISM=2
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 2 -run=TestAccAWSRoute_IPv6_To_LocalGateway -timeout 120m
=== RUN   TestAccAWSRoute_IPv6_To_LocalGateway
=== PAUSE TestAccAWSRoute_IPv6_To_LocalGateway
=== CONT  TestAccAWSRoute_IPv6_To_LocalGateway
    data_source_aws_outposts_outposts_test.go:66: skipping since no Outposts Outpost found
--- SKIP: TestAccAWSRoute_IPv6_To_LocalGateway (2.13s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	2.185s

aws/resource_aws_route_test.go

@@ -842,6 +842,48 @@ func TestAccAWSRoute_IPv4_To_LocalGateway(t *testing.T) {
 	})
 }
 
+func TestAccAWSRoute_IPv6_To_LocalGateway(t *testing.T) {
+	var route ec2.Route
+	resourceName := "aws_route.test"
+	localGatewayDataSourceName := "data.aws_ec2_local_gateway.first"
+	rName := acctest.RandomWithPrefix("tf-acc-test")
+	destinationCidr := "2002:bc9:1234:1a00::/56"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t); testAccPreCheckAWSOutpostsOutposts(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSRouteDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSRouteResourceConfigIpv6LocalGateway(rName, destinationCidr),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSRouteExists(resourceName, &route),
+					resource.TestCheckResourceAttr(resourceName, "destination_cidr_block", ""),
+					resource.TestCheckResourceAttr(resourceName, "destination_ipv6_cidr_block", destinationCidr),
+					resource.TestCheckResourceAttr(resourceName, "destination_prefix_list_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "egress_only_gateway_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "gateway_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "instance_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "instance_owner_id", ""),
+					resource.TestCheckResourceAttrPair(resourceName, "local_gateway_id", localGatewayDataSourceName, "id"),
+					resource.TestCheckResourceAttr(resourceName, "nat_gateway_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "network_interface_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "origin", ec2.RouteOriginCreateRoute),
+					resource.TestCheckResourceAttr(resourceName, "state", ec2.RouteStateActive),
+					resource.TestCheckResourceAttr(resourceName, "transit_gateway_id", ""),
+					resource.TestCheckResourceAttr(resourceName, "vpc_peering_connection_id", ""),
+				),
+			},
+			{
+				ResourceName:      resourceName,
+				ImportState:       true,
+				ImportStateIdFunc: testAccAWSRouteImportStateIdFunc(resourceName),
+				ImportStateVerify: true,
+			},
+		},
+	})
+}
+
 func TestAccAWSRoute_ConditionalCidrBlock(t *testing.T) {
 	var route ec2.Route
 	resourceName := "aws_route.test"
@@ -1553,6 +1595,56 @@ resource "aws_route" "test" {
 `, rName, destinationCidr)
 }
 
+func testAccAWSRouteResourceConfigIpv6LocalGateway(rName, destinationCidr string) string {
+	return fmt.Sprintf(`
+data "aws_ec2_local_gateways" "all" {}
+
+data "aws_ec2_local_gateway" "first" {
+  id = tolist(data.aws_ec2_local_gateways.all.ids)[0]
+}
+
+data "aws_ec2_local_gateway_route_tables" "all" {}
+
+data "aws_ec2_local_gateway_route_table" "first" {
+  local_gateway_route_table_id = tolist(data.aws_ec2_local_gateway_route_tables.all.ids)[0]
+}
+
+resource "aws_vpc" "test" {
+  cidr_block                       = "10.0.0.0/16"
+  assign_generated_ipv6_cidr_block = true
+
+  tags = {
+    Name = %[1]q
+  }
+}
+
+resource "aws_ec2_local_gateway_route_table_vpc_association" "example" {
+  local_gateway_route_table_id = data.aws_ec2_local_gateway_route_table.first.id
+  vpc_id                       = aws_vpc.test.id
+
+  tags = {
+    Name = %[1]q
+  }
+}
+
+resource "aws_route_table" "test" {
+  vpc_id = aws_vpc.test.id
+
+  tags = {
+    Name = %[1]q
+  }
+
+  depends_on = [aws_ec2_local_gateway_route_table_vpc_association.example]
+}
+
+resource "aws_route" "test" {
+  route_table_id              = aws_route_table.test.id
+  destination_ipv6_cidr_block = %[2]q
+  local_gateway_id            = data.aws_ec2_local_gateway.first.id
+}
+`, rName, destinationCidr)
+}
+
 func testAccAWSRouteConfigIpv4NetworkInterfaceAttached(rName, destinationCidr string) string {
 	return composeConfig(
 		testAccLatestAmazonNatInstanceAmiConfig(),