Add unit test coverage for db/repos/migrations

[nazarhussain]

Parent: #1484

Expected behavior

We need 100% coverage for db/repos/migrations

Actual behavior

Currently there is no coverage for this repo.

[karmacoma]

Reopened to address failing tests encountered by: #1637 and due to reversion to old promise syntax. Failing tests have been skipped for now.

[nazarhussain]

@vitaly-t Please looked the skipped tests specifically

https://github.com/LiskHQ/lisk/blob/1.0.0/test/unit/db/repos/migrations.js#L298

And suggest some pattern/approach to test functions which involving transactions and generator functions.

[nazarhussain]

@vitaly-t FYI, based on discussion with @karmacoma I had reverted your changes that was causing failures of test cases. Had created the respective issue vitaly-t/pg-promise#479

You do acknowledge that we love pg-promise and more importantly want to write clean and beautiful constructs introduced by you, but we have a principle that ugly code is better than beautiful but non-testable code.

As soon you fix that particular issue in pg-promise, will introduce the changes again.