Add shortcut methods for asserting one event or rejection

[alexander-yevsyukov]

This PR:

1. clarifies multi-tenancy status in API of creating BlackBoxBoundedContext.
2. addresses generics usage warning from ErrorProne.
3. adds shortcut methods for testing emitted events or rejections. So that instead of:

     context.assertThat(emittedEvent(TaskCreated.class, once()));

... it would be possible to write:

   context.assertEmitted(TaskCreated.class);

It is also possible to write:

   context.assertRejectedWith(SomeRejection.class);

[codecov]

Codecov Report

Merging #897 into master will increase coverage by 0.05%.
The diff coverage is 87.5%.

@@             Coverage Diff             @@
##             master    #897      +/-   ##
===========================================
+ Coverage     93.15%   93.2%   +0.05%     
- Complexity     3802    3808       +6     
===========================================
  Files           522     522              
  Lines         12541   12552      +11     
  Branches        710     710              
===========================================
+ Hits          11682   11699      +17     
+ Misses          629     627       -2     
+ Partials        230     226       -4

[alexander-yevsyukov]

@armiol, there is some strange anomaly of collecting rejections in BlackBoxBoundedContext. Please see my TODO note in the BlackBoxBoundedContext.assertRejectedWith(). We're getting two messages instead of one. If you can have somebody to look into this, I'd appreciate that.

PTAL to my changes in this PR. I decided to introduce them in relation to my current works on improving Kanban Board example.

[armiol]

@alexander-yevsyukov I have addressed your todo item. Please see this commit for more details.

[armiol]

@alexander-yevsyukov LGTM in general. Please see my comment — it's probably more convenient to discuss it in person.