Rewrite all tests using luatest framework

[Serpentian]

Currently a lot of tests are written using diff style. Let's port them to luatest.

Here're motivation points of doing this:

1. We need to develop the vshard's testing infrastructure for luatest.
2. Diff tests require a lot of code duplication.
3. Core readability is better for luatest tests.

This will dramatically decrease the time needed to write tests in the future.

[Serpentian]

Blocked by #107. Cannot test background fibers without manual grepping of logs until issue 107 is solved.

[Serpentian]

I don't think we should port tap tests to luatest, as first ones work much faster, doesn't require any infrastructure, which could be useful for us in the future, and their code is easy to read.