Add assertions for extraImgAttributes to TestsColumn

[Arne1303]

Description

Right now it is possible to check for extraAttributes on columns using

• assertTableColumnHasExtraAttributes()
• assertTableColumnDoesNotHaveExtraAttributes()

However, the same functionality does not exist when using extraImgAttributes on the ImageColumn. I added

• assertTableColumnHasExtraImgAttributes()
• assertTableColumnDoesNotHaveExtraImgAttributes()

to add that functionality.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[danharrin]

I'm not accepting new testing helpers like this, because then eventually we will end up with a testing helper for every single possible configuration method which is not practical.

Instead, I suggest using assertTableColumnExists() with a function to check if the column exists with the attributes that you specify