Integration tests should use Gateway/CoGateway interaction helpers

[abhayks1]

Context

Both MPP generator tool and Integration tests does stake & mint and redeem & unstake flow.
In MPP generator tool Gateway and CoGateway interaction methods has been refactored to helpers: *mosaic-contracts/proof_generation/helper/ **

For usability perspective and to avoid repeated code it makes sense for integration tests to use these helpers 📚 .
Refer discussion: #717 (comment)

Proposal

• Decide on ideal location for these helpers as currently they sit in proof_generation folder.

• Make changes in the helpers so that they are usable both for the tool 🔧 and integration tests.

• Update the caller code accordingly.

The ticket should be done after below PR is merged:
#717

[benjaminbollen]

#717 (comment)

Will create a ticket to integrate proof generator with truffle exec 👍

will wait for this ticket before moving this ticket forward.

[abhayks1]

Ticket created: #731