[terraform-ibm-transit-gateway] Set up and re-enable cross account test

[ocofaigh]

The cross account test was using an account that has been deleted, so in #446 the test was disabled to unblock the pipeline.

We need to re-enable this test, but lets consider the best way to do this. Perhaps we have the test create an enterprise sub account and a VPC inside it every time, but in order to do this we would need the ability to use an apikey from the newly created subaccount. Something that is being looked into as a feature in Q1 2024.

In the meantime, do we just create a new account for this test that has a VPC in it ready to go? Is there any account we already have that we can re-use?