[TASK] Refactor trustline test code to reduce duplication and test response body before status code and headers

[TASK] Use object for arguments to checkGetRequest and checkPostRequest

[TASK] Clean up trustline test code, change parameter names, function signatures

[TASK] Make arguments to testGetRequest and testPostRequest mandatory
Merge pull request #226 from clark800/feature/trustline_tests_refactor

[TASK] Refactor trustline test code
[FEATURE] Balances|Trustlines: Return ledger index (RLJS-127)
[FEATURE] Balances|Trustlines: Use last validated ledger by default
[FEATURE] Balances|Trustlines: Return ledger validated in response

Returns validated (boolean) if the ledger_index returned is validated or
not
[TASK] Balances|Trustlines: Check limit and ledger are valid uint32 and not empty string