Added tests. Fixed few minor issues found while writing tests.

Added tests. Fixed few minor issues found while writing tests.
Fixes #5 
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I'm not sure we should test it, since it's based on _ok which is private for HealthCheck class - not sure, but it sounds like _ok is something which shouldn't be private since it's used for custom checks. The same for details.
```
For now setting `_ok` and `_details` properties is a "protocol" for run() method for HealthCheck subclass. Thus, we have to fake these properties for tests. May be this design is not optimal, but I'm not sure we should change it in scope of this ticket.