Rename to DataObjectTest

[mfdj]

This PR suggests two things

1. Rename test to DataObjectTest because it tests DataObject class
2. use PSR-2 property-name for test subject

Rationale

1. I wanted a reference for how DataObject behaves and couldn't find a test. I was half finished writing my own test case before discovering that what I needed was ObjectTest? I think renaming it could help others.
2. PSR-2 coding style dictates that protected/private properties should not be prefixed with an underscore

[orlangur]

LGTM 👍 I'm surprised Go To > Test in PhpStorm cannot handle this.

Regarding leading underscore removal - please don't do it manually here and there, it would be more effective as automated change.

[mfdj]

@orlangur maybe I need to setup my phpstorm?

This is what I see on "go to > test" it prompts me to create a test?

Update

Ok weird, it worked after reloading PhpStorm — I think it just need to reindex or something. Thanks for the tip 👍

[orlangur]

@mfdj I mean I tried on mainline without your change and it didn't pick up wrongly named test properly.

[okorshenko]

@mfdj thank you for your contribution!
@orlangur thank you for review