Organize test structure, integrate go modules

[aacanakin]

No description provided.

[aacanakin]

Refactor postgres, sqlite and mysql e2e tests as well

[cdevienne]

I don't recommend using testify/suite.
Instead I suggest using subtests: https://golang.org/pkg/testing/#T.Run.
They provide the same advantages (setup/teardown, variables shared by a suite...) but don't require any additional tool, nor strange command line to run a single test.
As long as we don't have to support go < 1.7 I think we should use subtests.

[aacanakin]

Hi @cdevienne,

I see your point about using subtests. However, the current status of tests are horrible. They are really flaky. The reason I refactored the tests is because I'm planning a complete redesign. The new design will include your visitor implementation that is split into interfaces, table integration of visitor, package structure done correctly, and go idioms implemented correctly.

So, these tests are here for regressions that may appear on the new design.
Right now, testify's test suite & setup functions are enough for the project's needs. Setup function is called for each test case. I'll create an issue to evaluate & move to subtests.
I'll provide a release tag after this pr. This pr has no breaking change except glide. In your library, you can use the release tag.

[cdevienne]

The refactoring was long-needed, so it's definitely a good thing.
I am ok with postponing the switch to subtests