d/aws_route_table: Rework 'TestAccAWSRouteDataSource_basic'.

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSRouteDataSource_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSRouteDataSource_basic -timeout 120m
=== RUN   TestAccAWSRouteDataSource_basic
=== PAUSE TestAccAWSRouteDataSource_basic
=== CONT  TestAccAWSRouteDataSource_basic
--- PASS: TestAccAWSRouteDataSource_basic (141.12s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	141.166s

d/aws_route_table: Rework 'TestAccAWSRouteDataSource_TransitGatewayID'.

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSRouteDataSource_TransitGatewayID'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSRouteDataSource_TransitGatewayID -timeout 120m
=== RUN   TestAccAWSRouteDataSource_TransitGatewayID
=== PAUSE TestAccAWSRouteDataSource_TransitGatewayID
=== CONT  TestAccAWSRouteDataSource_TransitGatewayID
--- PASS: TestAccAWSRouteDataSource_TransitGatewayID (296.93s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	296.976s

d/aws_route_table: Add 'TestAccAWSRouteDataSource_IPv6DestinationCidr'.

Acceptance test output:

$ make testacc TEST=./aws/ TESTARGS='-run=TestAccAWSRouteDataSource_IPv6DestinationCidr'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws/ -v -count 1 -parallel 20 -run=TestAccAWSRouteDataSource_IPv6DestinationCidr -timeout 120m
=== RUN   TestAccAWSRouteDataSource_IPv6DestinationCidr
=== PAUSE TestAccAWSRouteDataSource_IPv6DestinationCidr
=== CONT  TestAccAWSRouteDataSource_IPv6DestinationCidr
--- PASS: TestAccAWSRouteDataSource_IPv6DestinationCidr (43.81s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	43.869s

Fix Acceptance Test Linting / terrafmt error.