Fix test asserts: reverse expected and actual params

[alexandear]

This PR fixes tests. It reverses passed parameters to the assert.Equal according to the signature (actual after expected):

func Equal(t TestingT), expected, actual interface{}, msgAndArgs ...interface{}) bool

Additionally, the PR enables testifylint linter to automatically detect this type of issues.

I have:

• Added tests covering the bug / feature (see testing)

[coveralls]

coverage: 75.124% (-0.04%) from 75.167%
when pulling 36769ca on alexandear:fix/test-assert-expected-actual
into bf4406a on 99designs:master.