Testing cleanup

## Overview

This pull request refactors all the tests:

1. Remove wrapping closure helpers (e.g. `withOpenDB()`).

2. Replace Testify with [a simple testing impl](https://github.com/benbjohnson/testing).

3. Move tests to the `bolt_test` package.

This helps the tests to be more maintainable (IMO). A more detailed description about the reasons can be found here: https://medium.com/@benbjohnson/structuring-tests-in-go-46ddee7a25c

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/cc @mperham because we were chatting about this briefly on Twitter. :)

/cc @snormore @mkobetic because they contribute to Bolt. :+1: