Updated: Comment #0
Problem/Motivation
FieldableDatabaseEntityStorageTest.php contains the class ContentEntityDatabaseStorageTest. Per the PSR-0 standard it should be called ContentEntityDatabaseStorageTest.php
This is an artifact of #2188613: Rename EntityStorageController to EntityStorage (I think, didn't check.)
Proposed resolution
Rename the file to ContentEntityDatabaseStorageTest.php
For extra credit remove the unused use statement for EntityType (line 10) as well.
If you are familiar with PhpUnit tests or want to become it, you can see what other tests do with @coversDefaultClass and @covers and apply that here as well. That would really be extra, extra credit then.
Only the rename is required though, no stress! :-)