r/aws_ec2_transit_gateway_peering_attachment: Get acceptance tests pa…

[j-nix]

…ssing.

Implement @ewbankkit's work for transit gateway peering:

hashicorp#11162 (comment)

• The resources were left dangling because the Config value was not set on the import test step as required for cross-region acceptance tests and documented in the Contributing guide
• The test case was actually failing because of a resource name mismatch (.example vs. .test)
• Because of the dangling resources I ran the test sweeper and add it as a dependency to the aws_ec2_transit_gateway test sweeper
• I made peer_account_id optional (defaults to current account ID) as for VPC peering connections and also made all the other attributes (except tags) ForceNew: true which means that the Update method only deals with changes to tags
• Tidied up various other things on the way