Improve ignored unit test annotation strings

[edchapman88]

Tests across the repo are annotated with 'ignore' if they require a running ION server (ION node, MongoDB, IPFS, Bitcoin RPC) or running a trustchain-http server (or both).

eg.

    #[test]
    #[ignore = "integration test requires ION, MongoDB, IPFS and Bitcoin RPC"]
    fn test_example() {
          ... 
    }

TODO:

• Standardise these test strings so they are consistent across the repo.
• Could they be string constants defined and maintained in a single place?