Add unit tests across backend package

[nitishm]

Need to add unit tests for the backend server to test all the attack options plus all the additional logic in the internal/vegeta/attack.go, internal/vegeta/report.go and internal/vegeta/store.go among the other files and packages.

[andrysds]

@nitishm I'll try to work on this issue.

[nitishm]

Awesome !!!

[nitishm]

@andrysds Heads up. I have changed the entire implementation from go-swagger to gin-gonic. Most, if not all, of the backend code has been refactored. With the new structure it should be much easier to add dependency mocks for unit tests.

Let me know if you are still interested in working this issue.

[nitishm]

NOTE: Unit tests should use mockery/testify to create mock interface implementations.