Create functional tests to test retrieving transactions from the transaction index

[TheQuantumPhysicist]

The test should include:

1. Test enabling the transaction index flag as in The transaction index flag should not be aggressive to insist the user enable it to use it every time #1154
2. Test retrieving transactions that are in the mainchain (both in json and hex encoded format, which is done in Enable retrieving both transactions and blocks in json format #1155)
3. Test retrieving transactions that are not in the maincahin anymore and expect that to fail

[TheQuantumPhysicist]

Transaction index is gone. We now use the API server for such things.