Add Black Box Context tests guide

[yuri-sergiichuk]

While we believe that having a fully tested code is essential, we should consider adding a guide and/or reference cases of the tests written using the Black Box Context (BBC).

The guide should:

• cover Dos and Don'ts of the BBC testing;
• cover a way to structure tests and test cases;
• cover configuration of the BBC tests;
• show how BBC tests should co-exist with unit tests.