Splitting up assertions in unit tests

[Rahmon]

Is your enhancement related to a problem? Please describe.

Describe the solution you'd like
Some unit tests are testing different cases in one method. For example, testGetDocuments tests retrieving documents, retrieving a document that doesn't exist, and passing an empty id to retrieve a document.

When this test doesn't pass, it's not clear exactly which case have failed:

1) ElasticPressTest\TestElasticsearch::testGetDocuments
Failed asserting that 2 matches expected 3.

So, we could try to split up assertions a little to have more focused tests.

Designs

Describe alternatives you've considered

Additional context