Test for Substrate Fungible class

[wainola]

Provide test for Substrate Fungible class.

Implementation details

• Define mocks if necessary for the different methods or functons being used
• Test cases for Fee and transaction
• failing and success conditions
• Refactor current mock implementation of ApiPromise to something that is present globally when running test, so we can just pass parameters

Testing details

Acceptance Criteria

• Substrate Fungible class and related functions are tested and all test are passing

QA Details